Problem solved. The solution was to add a Public Folders database using the EMC (Even though I don't use or need Public Folders).
It appears that BES Express 5.0.2 SP2 can no longer run without Public Folders in Exchange 2010 SP1, despite the fact that I had the "Ignore No PF" registry key DWORD set to 1 in HKEY_LOCAL_MACHINESOFTWAREWOW6432NodeMicrosoftWindows Messaging Subsystem
See this page. http://docs.blackberry.com/en/admin/deliverables/14347/Config_Exchange_10_run_wo_public_folders_963029_11.jsp
BES Express 5.0.2 SP2 was working just fine running without Public Folders so installing Exchange 2010 SP1 definately broke it.
What tipped me off to finally resolved this was that the BES Utility IEMSTest was failing during the CDO Calendar test. That made me take a closer look at this error in my RPC Client Access log file.
"RopHandler: Logon: [RopExecutionException] A user doesn't have an associated Public Folder database or it's not connectable (Mailbox DB Guid = 254ff931-96f4-4aed-85e7-8968799a4311, PF DB Guid = 00000000-0000-0000-0000-000000000000). Error code = LogonFailed"
The Guid with all zeroes was probably the cause of the "[InvalidOperationException] Nullable object must have a value." error that was also in the same log file.
Anyhow, my total time wasted trying to resolve this : 8 Hours
Thanks, Microsoft!