I'm seeing a lot of posts from people who have more than one iTunes account on a computer running into this issue.
http://support.apple.com/kb/HT4627
From the article (emphasis mine):
When you turn on Automatic Downloads or download past purchases on an iOS device or computer, your device or computer will be associated with your Apple ID. Your Apple ID can have up to 10 devices and computers (combined) associated with it. Each computer must also be authorized using the same Apple ID.
Once a device or computer is associated with your Apple ID, you cannot associate that device or computer with another Apple ID for 90 days. You can view which devices or computers are currently associated, remove unused devices or computers, and see how long before they can be associated with a different Apple ID from the Account Information page in iTunes on your computer:
Also, Mr. Pink - I miss the pink first letter in your sig.