My previous post was incorrect in that it said "This will be fixed in the upcoming Service Pack 2 for 10.0". Unfortunately we were not able to get our completed fix into the upcoming Service Pack 2 for 10.0, so you'll still encounter this issue when trying to open a web map created after March 2 2011 in ArcMap. Sorry about the issue. We hope to be releasing a fix for this for 10.0 users soon.