We are actively working on 6.5 compatibility for the next release. So far our testing has been limited by having no access to WM6.5 on a real device having only the SDK Emulator to test against. We have observed a couple of extension points which previously worked are no longer showing when run on 6.5. There have been a few incidences where the application would stop responding following a menu extension tapped. There is no documentation from Microsoft indicating there should be any changes to any of this.
What particular devices have you had problems with? Any more detail on the issue - are the menu items not showing at all, not responding to taps etc...
Peter