If you downloaded WordPress 2.1.1 prior to March 2nd, you need to download the upgrade immediately. A cracker managed to mess with the code of the download file.
If you’re lazy like me, and still have a version that is not 2.1.1, you’re safe. Or if you downloaded it after March 2nd, you’re safe. But for about four days prior to the 2nd, you downloaded a poisoned copy of WP.
No word on what the cracker wanted to do, but I’m guessing it wasn’t send you a birthday card.