We had a bit of an issue this week with an Exchange 2003 server and AVG 2011 - it appears that an update in 2011 with the "Online Shield" caused issues with Active sync on the server (which meant that the mobile devices stopped receiving e-mail).
The details in the Event Log were:
Exchange mailbox Server response timeout: Server: User: Exchange ActiveSync Server failed to communicate with the Exchange mailbox server in a timely manner. Verify that the Exchange mailbox Server is working correctly and is not overloaded.
It took us a few hours to figure out what was going on, but we narrowed it down to AVG, and by turning off the "Online Shield" (screenshot below) this fixed the issue - we hope this article helps other users with similar issues.