We recently found a bug in the way Exchange 2010 calls into your EWS Push Notification endpoint. You may see errors on the Exchange server indicating that it received HTTP 500 errors when calling into your client. Basically, the SOAPAction header is not being sent along with the request. I’ll update this post when this issue is resolved.
[Update] 3/5/2010 – Update Rollup 2 was released containing the fix for this issue.