I have been using my original iPhone with my Sennheiser MM400 for a long time...no problem (well, actually, from time to time the Sennheiser would lose connection but I always figured it was my fault for getting too far away from the phone or something).
This week I got my new iPhone 4s.
The first interesting thing I noticed is that it had some difficulty connecting to my Ford Sync bluetooth. I finally made that work.
However, it will not recognize the MM400 at all. I have tried several of the ideas in this thread with no luck. I am running the latest ios.
I was able to pair the iPhone to my Motorola H780 (with siri off...have not tried with siri on).
I suspect that Sennheiser's bluetooth may be partly to blame, but there may be some shared blame with Apple as well.