It looks like there is some unknown exploit that is being used to hack into WordPress sites en-mass.
The following threads and sites will tell you more about it and how you can find out if you are effected.
http://wordpress.org/support/topic/385477
http://techcocktail.com/home/2010/04...earch-engines/
Note: third link removed - - see post #2
update: seems this site was itself hacked - the author believes it is clean now the site is www christopherspenn com/2010/04/07/find-the-latest-wordpress-hack/
http://www.themelab.com/2010/03/01/d...earch-engines/