Cheaper alternative with bluetooth
I have been using the following with my AM/FM radio, my mp3 player, and now my Zumo to get Bluetooth in my helmet.
I changed from helmet speakers which I really couldn't hear to earbuds which can be easily heard.
The Jabra Model A120 dongle ($24- Amazon) converts anything out of the players, including the Zumo, into stereo.
I have the following velcroed to the bottom edge of my helmet- (about the size of flashdrive) Satechi Wonder Stereo Headset ($69.99-Amazon). It paired up with the Jabra right away and once you turn both on, pairs up fast each time.
It came with a combination microphone/earbud set and says it will pair up to both speak or listen. I don't have an interest in using my phone with it (non bluetooth phone) so I can't speak to that, but it works great with the mp3 and XM features of the Zumo.
Also, both pieces charge with a mini USB port. The Zumo 12v and 110v adapters charge either one.
I found the info to buy these on this forum as well as the Zumo forum. For under $100, I'm happy.
So far so good!