When I go to my back office orders tab (something I do often), I get the following error:
Warning, there is a shipping problem in the shopping cart!
Time: Thu Jun 14 13:12:17 2012
USPS Message: Error Information: The Destination ZIP Code you have entered is invalid.
It doesn't seem like a main order page error message.
For what it's worth, we upgraded last week from v11 SP1 to SP2.