Despite the easily-resolved RPC encryption problem being the "top support issue", the lack of UDP notifications is a real killer if you can't use cached mode to overcome it. Unless you are happy for your users to have to wait random amounts of time between 5 and 10 seconds for any changes in their mailbox to become apparent, and deal with error messages if they attempt to force the issue, Outlook 2003 in online mode with Exchange 2010 is a complete non starter and should never have been considered "compatible".
Extra icing on the cake is not being able to roll back to Exchange 2007 if you have installed Exchange 2010 in your environment.
Microsoft
