There is a hack going around that may or may not affect wordpress 2.5.1 blogs, but definitely affects all older versions. (There are known 2.5.1 blogs that are infected, but the general consensus is that they got hacked before being upgraded and carried the upgrade through.)
It is a very tricky problem. Basically, if somebody gets a search result in google and clicks through to your site, they are immediately forwarded to a spammer's site instead. A cookie gets dropped in the process, though, so that it doesn't happen a second time. This prevents site owners from noticing anything strange or troubleshooting it other than that google traffic begins to dry up. It can also lead to your site being temporarily delisted.
To see if your site has been hacked, clear the cookies on your blog (or use a computer that has never visited your blog) and do a search that returns a result that links to your page. Click the link. Did you wind up on your site? If so, you aren't being exploited right now and may not have been hacked. (Then again, you may have been.) If you do go to a scammer's site, you've definitely been hacked.
JD Roth has put up some good instructions for fixing your site, which you can read here:
Patching the WordPress AnyResults.Net Hack ∞ Get Rich Slowly
There is also a video that you may find helpful on YouTube:
YouTube - Remove Wordpress Spam Redirect