Microsoft published a KB article that confirmed the problem: meeting request that you send from an EWS application is in plain text format instead of HTML format when an attendee opens the request by using Outlook in online mode
The fix? Patch your servers! Here's the link: Update Rollup 3 for Exchange Server 2010 Service Pack 2 (KB2685289)
We patched our dev server this weekend and confirmed it works!
(PS: we're tagging this as an API bug, but it really isn't.....