I never ran into this issue until few days ago. Migrating Microsoft Exchange servers to Google Apps often require definition of forwarding rules. Most often you'll want to auto-forward emails sent to Exchange accounts which were already migrated to Google Apps by people who wasn't migrated yet (co-existence mode).
It seems like Exchange 2007/2010 doesn't allow (by default) setting forwarding rules for external domains (and your Google Apps is considered external domain). I think I never ran into this before, because all of the Microsoft Exchange servers I migrated were upgraded from prior versions (like 2000/2003) rather than fresh/new installs.
Fortunately, there is an easy way to fix it. Go to PowerShell and type this command:
set-remotedomain -identity Default -AutoForwardEnabled $true