This is a quick warning: there is no WordPress 2.6.4 release; if you see notices that you need to upgrade to this version check them very carefully. A fake wordpress website appeared earlier today at; it looked the same but offered a fake 2.6.4 release containing a backdoor. 

Craig Murphy has all the details. The fake site is offline at this moment, but this incident serves as a good reminder to keep your software up-to-date, and to install your updates only from trusted sources!