Some of our readers might be interested in checking out the recently established Secular Right blog, which seeks to demonstrate that atheism and secularism are compatible with being on the political right. Among the contributors are Heather Mac Donald of the Manhattan Institute (who wrote a much-debated article on atheism and conservatism in 2006), John Derbyshire, Walter Olson, and Razib Khan.
Although one of the four contributors (Olson) is more libertarian than conservative, the main focus of the blog seems to be on the latter. After all, few doubt that one can be both an atheist and a libertarian. Many of the most influential libertarian thinkers of modern times were atheists or agnostics (e.g. - Milton Friedman, F.A. Hayek, and Ayn Rand). Although there are also some highly religious libertarian intellectuals, including some of my co-bloggers here at the VC, few if any libertarian theists doubt that an atheist can be just as much a libertarian as they are.
With conservatism, the issue is much more contestable. On balance, I think it logically possible for a person to hold conservative views on political issues for reasons that have no connection to religion. However, many conservatives do seem to believe that there is a close connection between their political views and their religious ones; some also hold that one can't be a "real" conservative without also being religious. Prominent conservative writer Richard John Neuhaus wrote a well-known 1991 article in First Things, "Can Atheists be Good Citizens?" arguing that atheists not only can't be conservative, but cannot even be "good citizens" at all (unfortunately, I haven't been able to find this famous article online, perhaps because it dates to the pre-internet era). Of course close connections between religion and political ideology can be found elsewhere on the political spectrum as well, as witness the example of Marxist "liberation theology." In the modern US, however, conservatives are more likely to emphasize such connections than either libertarians or liberals.
Ultimately, whether conservatism is compatible with atheism depends to some degree on one's definition of "conservative." If to be conservative means to hold right-wing positions on various political issues, then atheists can be just as conservative as anyone else. If, on the other hand, conservatism is defined in part by having a religious foundation for one's political views (as Neuhaus, among others, contends), then they can't.
UPDATE: As commenters point out, Neuhaus' article arguing that atheists can't be good citizens is available here.