Has anybody managed to find a workaround or resolution to this issue yet?
I can also confirm this problem comes occurs on 2 client computers running Win7Pro x64. Both have Office 2010 installed with all available updates. It's not the sender. This appears to be a Microsoft Outlook 2010 issue. I will contribute, however, for the record that both of these computers are running AVG Free.
Disable the AVG addin and test. Also opening Outlook in Safe mode. (Hold Ctrl as you click on the Outlook icon to start in Safe mode.) If the situtation is the same as p1nt's, something in outlook is actually changing the message source so you need to disable all addins and enable them one at a time to test.
We know AVG 2011 has some 'issues' with outlook - it may have more issues than previously known and for this reason, i would start with that. If it doesn't help with new messages, use safe mode and see if the problem returns. (AVG itself is fine, its the addin that enabled email scanning that is the problem.)
Note that messages already corrupted will not be fixed - but new messages should be ok.