We are seeing the following event ID on our single Exchange 2010 server several times a day:
Log Name: Application
Source: MSExchange ActiveSync
Date: 2/9/2011 12:59:35 PM
Event ID: 1100
Task Category: Requests
Exchange ActiveSync device requests for your users are being blocked. This problem frequently occurs when the HTTP OPTIONS method request isn't allowed by the firewall. Please check the firewall that filters requests in front of your Client Access server and the Microsoft-Server-ActiveSync virtual directory.
Mobile email access *is* working correctly. We have disabled the firewall on the Exchange server, but are still seeing the error.