It's not really rocket science... and it can be done fairly cheaply, esp if you have a smart phone ( or even a half-smart phone like me ).
I use a tracfone GSM... runs me $7-8 per month ( and I can replace it for $20 ). This gives me Intercom( with group too ), music, phone. The phone links as HST and A2DP so the 8gb card in the phone has losts of music. As GPS, it for just visual, no audio.
If I want GPS/MUSIC then it is a A2DP dongle plugged into the GPS and I link it before the phone ( so only the phone links as HST ).
The wife has a CDMA tracfone ( verizon )... the advantage is she has Voice dial thru the Sena, I don't.
With the newer V5 Sena software, it allows you to share music with another , see ;
http://www.youtube.com/watch?v=GJ9iQwdq1vE
It does work, and work well but still has it's limits!