The probelm I've found with Exchange Gmail Sync is being looked at by Google now, you can follow it over here:
Using 3.0 on iPhone with Push configured on Calendar and Contacts causes inbound calls to be dropped / not connected. Diagnosed with o2 in the UK. Approx 1 in 3 calls dont get through to the handset - its as if the handset is "busy" and cant take the call. Callers are divereted to voicemail etc. Turn off push on contacts and calendar and all works.Tryed another handset same problem.