I have a similar issue as described in the
http://forum.ppcgeeks.com/showthread.php?t=29368
I have an appointment (Outlook 2007) during "work hours" to put the phone in vibrate mode. During this time, an incoming call vibrates then pauses, then vibrates again in a cycle. However, outside the "work hours", it vibrates constantly (no pausing) and no ringing. I thought my speaker was broken, but outside the "work hours" I can successfully go into the ringer settings and hit the play button and hear the speaker. I can also go into the dialer and hear the button pushes.
I have no idea what could be the problem either.