Creationists often argue against any scientific claim that is based on uniformitarianism.
A geology text book provides a good definition of that term:
Uniformitarianism – The belief or principle that the past history of the earth and its inhabitants is best interpreted in terms of what is known about the present. Uniformitarianism explains the past by appealing to known laws and principles acting in a gradual, uniform way through past ages.
This is really not a concept that is difficult to understand. We can apply the same idea to many things that we see in everyday life.
Let’s say you are standing next to a highway and a car drives by you at a speed of 60 miles-per-hour (or one mile-per-minute). That is what we know about the present.
Based on that, what is your opinion about where that car was one minute ago? You didn’t see it one minute ago, but surely it is very reasonable to guess that the car was one mile further up the highway.
Furthermore, it is reasonable to guess that ten minutes ago, the car was ten miles further up the highway.
As you go further and further back in time, the process of guessing where that car was still works but it becomes less and less likely. Since most trips by car are less than 30 miles, the odds that one hour ago the car was 60 miles up the highway are probably less than 50% though that is still a valid possibility. If you go back a full day – 24 hours – it is quite unlikely that you can simply multiply the time by 60 mph and get the likely position of the car at that time. It is likely that the trip wasn’t that long, and even if it was the driver surely stopped along the way to eat and purchase gas.
The point – we can look at current data and make reasonable conclusions about the immediate past. But as you go further and further back in time the odds that the current data is consistent throughout that time frame.
When discussing processes that affect the Earth’s surface, they take place in “geological” time. That means many millions of years. But when dealing with very long periods of time, geologists recognize that it is probably incorrect to simply assume that current processes were operating at that same rate though that has not always been the case.
Wikipedia provides this bit of history about uniformitarianism:
The concept of uniformity in geological processes can be traced back to the Persian geologist, Avicenna (Ibn Sina), in The Book of Healing, published in 1027. Modern uniformitarianism was formulated by Scottish naturalists in the late 18th century, starting with the work of the geologist, James Hutton, which was refined by John Playfair and popularised by Charles Lyell's Principles of Geology in 1830. The term uniformitarianism was coined in 1832 by William Whewell, who also coined the term catastrophism for the idea that the Earth was shaped by a series of sudden, short-lived, violent events.
Charles Lyell’s work influenced Charles Darwin.
Here is a discussion on the current views of uniformitarianism from the Talkorigins discussion forum:
Originally, uniformitarianism included a belief in a rough uniformity of rate of all natural processes. For example, the time required to deposit a given sedimentary formation would have been estimated by using observed deposition rates for similar present-day formations. This sort of strict uniformitarianism has been out of favor for a very long time. Even when it was in favor, it was recognized as being a convenient simplification at best.
However, uniformitarianism has always encompassed much more than that. For example, the uniformity of existence of natural processes, resulting from a uniformity in the basic laws of physics. Even though rate, intensity, and relative importance of natural processes may change over time, the processes observed in the present had been operating in the past as well. Those past processes left traces that look like the traces which those same processes are observed to leave today. "The present is the key to the past," is one way that it is stated.
Modern geologists use the term actualism for these concepts. You may think of actualism as being equal to the modern definition of uniformitarianism, or perhaps as the subset of uniformitarnianism which is still accepted. The overwhelming majority of modern geologists accept actualism -- meaning that they accept a number of the components of uniformitarianism as it was originally defined. You'll find a similar definition of actualism in most introductory geology texts, for example p. 521 of Cooper et al.'s A Trip Through Time, and p. G-1 of Dott and Prothero's Evolution of the Earth.
To the extent that geologists have rejected uniformitarianism, it's of little comfort to the young-Earth creationist cause. The evidence supporting actualism quite clearly rules out the history of the Earth which they desire (for religious, rather than scientific reasons) to support.
In summary, modern geologists don’t really believe in strict uniformitarianism.
The reason that this subject is relevant to creationism is because creationists insist that the Earth’s geological features are due primarily to the Biblical flood of Noah. They embrace the lessening influence of uniformitarianism and say that it is evidence that the concept that the flood is a real event is gaining acceptance.
Here’s how one creationist put it:
> Apparently you still have not realised that Darwinian
> gradualism with its doctrine of "Uniformitarianism"
> has been substantually replaced by many evolutionists
> with "Catastrophism" -- much to the satisfaction of
> Creation scientists, who have been catastrophists all
But modern geologists are not catastrophists. They are actualists. Surely they recognize that catastrophes have occurred. There have been meteors that have struck the Earth with devastating effects, there have been large regional floods, there were Ice Ages and the like. It has always been the case that geologists understood the ideas of local floods and the other catastrophies. There really is no major paradigm shift here.
But there has never been a global flood.
The idea that giving up the concept of uniformitarianism implies acceptance of the idea of a global flood is absurd.
Moreover, and very ironically, many creationist claims are, at their core, uniformitarian arguments!
For example, one evidence for a Young Earth is the decay in the Earth’s Magnetic field. That decay has been measured consistently for the last 30 years.
Here’s how Answers in Genesis describes it:
`In the most accurately recorded period, from 1970 to 2000, the total (dipole plus non-dipole) energy in the earth's magnetic field has steadily decreased by 1.41±0.16%. At that rate, the field would lose at least half its energy every 1500 years, give or take a century or so. This supports the creationist model that the field has always been losing energy—even during magnetic polarity reversals during the Genesis flood—ever since God created it about 6000 years ago.”
So they are taking present readings (at least those for the last 30 years which is no more than an eye blink in geological time) and extrapolating that back through “past ages”. Which, if we refer back to the very definition of “uniformitarianism”, is just that! Effectively creationists using uniformitarianism as an argument each and every time that it suits them.
Obviously consistency is not one of the facets of creationism.
 Definitions from Essentials of Earth History, by W. Lee Stokes, Prentice-Hall, Inc., 1982
 http://en.wikipedia.org/wiki/Uniformitarianism_(science), referenced on February 25, 2009
 http://www.talkorigins.org/origins/feedback/jan01.html, referenced on February 25, 2009
 http://www.answersingenesis.org/creation/v20/i2/magnetic.asp#1f6, referenced on January 25, 2009