http://www.ducatimonsterforum.org/index.php?topic=35667.0BlueAnt F4 here... works great. Tho, I think Interphone isn't making them anymore.
As for the whole "focus" discussion... I find that having the music while riding actually helps me focus on my riding, especially on longer travels.
I agree that phone calls can be distracting when operating ANY motor vehicle. Phone calls can be ignored... same as in a car. But to talk about the functionality of it... I have taken calls while riding (and made calls, via voice), and have had no issues. One button connects the call, and music automatically starts back when the call ends.
I agree with you in past rides I would get fatigued and lose focus just because my mind wasn't being engaged enough. I haven't rode listening to anything yet but when I get to the states in Sept I will totally try it and see how I feel about it.
One of the main reasons I want Bluetooth audio in my helmet was to link my GPS up to it and talk to my GF on trips. Music was just a secondary thing and wasn't even important to me but if I'm alone then music would be something I would listen too more often then nothing.
As far as a phone all I have is a 3g iPad and not a cell phone. I prefer the large screen size because of my very large hands and want to just carry one device. I use Skype or Google Voice to make or receive calls and would use that with a Bluetooth helmet. I would rather someone text me or sent me a IM via Gtalk and even then I would respond when I felt like it if at all.