Apple's app store looks to have been compromised this morning as a number of the top books in the app store belong to the same publisher. More importantly, a number of users in the US and UK have reported being charged for unauthorized app purchases. It is unknown right now exactly how many user accounts have been compromised, however it is strongly encouraged that all users change their iTunes account password immediately. Users have been posted to Twitter than they have been charged as much as $600, usually in block of $40-$50 purchases. While we expect Apple to refund all fraudulent charges, it doesn't make having the account hacked and funds withdrawn any better.
The report first surfaced when a couple of app developers first noticed that they had been bumped from their app store ranking, to be replaced with a mysterious publisher with no website. Now, 40+ of the top 50 "iTunes top paid books" are from the same publisher.
Rather than wait for your emailed receipt to see if you have been charged for anything, you can view your recent purchases in iTunes. For information on how to do that, see Apple Related's guide.