we upgraded one year ago from Exchange 2007 to 2010.
If I want to give Permission in Outlook 2010 to a mail-activated group (security group or not) i get a failure.
See the groups, that can't be used have a red sign.
Old groups from Exchange 2007 are Ok, unless I give the group a managing account (managed by under group information).
Then exchange says: upgrade to new version and after that I'm not able to use this group to give permission anymore.