yeah i posted an answer that worked for me..i changed the ringer type for the incallbuzz value in the registry.
http://forum.ppcgeeks.com/showthread.php?t=41178
HKCU/ControlPanel/Sounds/InCallBuzz
I changed the Sound value from Notify.wma to Default.wma
So now it just beeps 3 times when a text comes in while on a call.
you could probably delete it and you wont get an audible notification during a call.