It is worth making two points. First, the term
uniformitarianism is no longer current within geology, though the principles are very much alive and well.
Second,
uniformitarianism does not mean what most creationists think it means. It is not a synonym for gradualism. It means that the array of forces active on our planet today have been active throughout its history. It definitely does not rule out catastrophes. For example, an asteroid strike is possible today, and an asteroid strike was possible millions of years ago.
Wikipedia has a good article on
uniformitarianism.
--Percy