Evolution is indeed a theory, but we need to remember that a theory is not "something tentative or unsubstantiated." A theory is an explanation of known facts, and theories which explain a great many facts are on a pretty solid footing. Evolution is one of those theories. While it may not be quite as certain as, say, the laws of thermodynamics, it's certainly as well-supported as atomic theory, or the periodic law of chemistry. For more on the evidence for evolution as a scientific theory, spend some time with the Talk-Origins archive.

Disclaimer: I take no responsibility for the anti-theistic content of the so-called "humor" on the T-O site. By and large the scientific articles are well-balanced and reasonably polite; some are written by Christians and other theists.

Discussions of creation tend to lie in the areas of theology and philosophy, rather than science, because science, as Francis Bacon tells us in his 1605 work The Advancement of Learning, is concerned only with immediate ("efficient") causes and cannot detect ultimate causes, at least not unambigously.

What this means is that creation and evolution are not competing ideas; one is a theological concept and the other is a scientific concept. They work in different spheres and there is no scientific reason that they cannot be considered mutually consistent. Those who claim that evolution and creation are incompatible have theological or philosophical (that is, metascientific) reasons.

For more on this, you might consult a number of books and articles. Some are on the web, some are not.

• The Statement on Teaching Evolution of the National Association of Biology Teachers bears directly on your question.

• Pennock, R. 1999. Tower of Babel: The Evidence against the New Creationism. Cambridge, MA: The MIT Press. Pennock is a philosopher of science who clearly shows that creation itself is beyond the scope of science, and that "scientific creationism" is based on a flawed epistemology (the philosophical study of "how we know"). My review of this book should appear soon in the Journal of College Science Teaching.

• The National Academy of Sciences maintains an evolution bibliography which is rather slanted toward a non-theistic interpretation of the data. Of the books listed, I recommend
  • Numbers, R. 1993. The Creationists: The Evolution of Scientific Creationism. Berkeley, CA: University of California Press. This book discusses the history of the Creationist movement in the United States, indicating (a) why evangelicals in this country have become so committed to a literal reading of Genesis 1-2 and (b) why the controversy is such a non-issue among European evangelicals.
  • Ruse, M. 1996. But Is It Science? The Philosophical Question in the Creation/Evolution Controversy. Amherst, NY: Prometheus Books. Michael Ruse is not a theist, but is very fair.

• This three-part review of Of Pandas and People, which discusses "intelligent design theory" and creationism in some detail, is from the website of the National Association of Biology Teachers and is written by Richard P. Aulie, an evangelical Christian.

• The Science-and-Christianity E-mail List home page (http://www.shawangunk.com/scichr/ link edfunct as of 8/23/2006; try http://www.bluffton.edu/~bergerd/scichr.html) contains a number of essays and book reviews related to your question. It also contains many useful web-links.

• The Affiliation of Christian Geologists does, too.

• I have written a few essays relating to this topic (and others).

• a search of our web site with the keywords "creation evolution" (limiting to whole words) turned up a large number of questions and answers which bear on your question.

Disclaimer: The contents of this answer may or may not be the opinion of anyone else associated with MadSci Network. To the extent that opinions are expressed here, they are my own; to the extent that facts are cited here, they are as accurate as I can make them.