I've received several emails from folks who have been successfully using my Outlook Calendar Provider and Outlook Contact Provider with Outlook 2003.  Unfortunately, they upgrade to Outlook 2007, and their solutions break.  This appears to be due to Outlook 2007 being more strict about the response it gets back from the Web service than was its predecessor.  The good news is that Patrick Creehan, a support engineer on the messaging team at Microsoft, has already come up with a fix.  On his blog, he details the few changes you'll need to make to the existing calendar provider framework to get it working with Outlook 2007.  Thanks, Patrick!