Despite the documentation, CUPS / CUPC Outlook calendar integration WILL work while Outlook Web Access is using forms based authentication (if you see a pretty web form when logging into OWA vs. a Windows form requesting a username and password).
One problem is while setting it up and troubleshooting CUPC, the errors returned from Exchange during the CUPC / CUPS calendar communication can be based on results from OWA / IIS, and not Exchange. You may get errors that are either too general to be helpful or misleading (i.e. 440 Timeout).
By disabling FBA in Exchange (and / or IIS), you can restart the Presense engine and collect real errors rather quickly.
I found Exchange was looking for a DOMAIN\USER Presence Outlook Gateway User loggin name format (vs. simply USER, or cn=USER, ou=DOMAIN...). TAC may suggest any variation of them without really investigating the root requirements from Exchange. One mystery problem is that an incorrect authentication method CAN work for a long enough time to make you think your exhaustive list of calendar testing is complete, and then simply stop working an hour later.
Drop FBA, verify what Exchange is looking for re: loggin methods, and cross your fingers.
When you decipher the root cause of your calendar integration failure, you can re-enable FBA on OWA.