A worm infected Twitter on Saturday April 11, 2009.
The worm seems to have originated at the StalkDaily.com site, as it seems that it's main purpose is to direct people there. Twitter warned people against visiting the site or linking to it.
The worm sends spam tweets from the infected person's account that direct others to the StalkDaily site. Simply visiting the profile page of an infected user can lead to one's own profile getting infected.
If you have been locked out of your twitter account due to this worm, you need to do a password reset as Twitter has locked some infected accounts.
Twitter claims to have now fixed the security hole that enabled this worm, and that no passwords, phone numbers, or other sensitive information were compromised as part of this attack.