It would appear that problems with Outlook complaining that something is "Not Implemented" are more widespread than I thought when I encountered the issue. Someone on the Technet forums has encountered the "Not Implemented" issue, with what looks like a slightly different set of circumstances, and another person on the Zimbra forums is also encountering the error.

I mentioned the issue, along with my solution, over on the the Office 2010 Engineering blog, but have yet to see anyone else comment there that they've encountered the issue, or any MS bods acknolwedge that it's a known issue. I cross-posted the comment to the Outlook blog so anyone who encounters the problem would stand a better chance of getting to try my solution, but it looks like the comment wasn't released :(

I have to say, this is the only bug I've found with Outlook 2010 so far, though!