I see there is a option for MWI in obi110. Can Google Voice server toggle that bit for obi when there is new voice mail?
It would appear that Google Voice doesn't support MWI (not very surprising):
http://www.google.com/support/forum/p/voice/thread?tid=5a31ebfd558ad2de&hl=en
There is a solution for this in the upcoming 1.2 f/w release, due out in 1-2 weeks.