Okay, I have previously connected the iPhone 4 to the Zumo 550 via bluetooth, and answered/dialed called using the Zumo's touchscreen. I have the hockey puc antenna and listen to XM through the Zumo as well. Up till now, all of this audio has been routed through a StarCom1 hardwired to my helmet.
So, if I use the hardwired audio coming out of the Zumo (both nav and music outputs) and plug that into a BT dongle, pairing SRC to the dongle, I should have nav prompts, phone and XM/MP3 in stereo, right?
As stated above, I was a little dismayed that the SRC only allows BT connection to one device at a time. It's definitely the slickest, most streamlined helmet audio system I've found, but that's an annoying little inadequacy.