a) on headsets: 99% of my calls are done via wired headset, almost never, unless I'm not expecting it, do I talk with the handset to my head.

b) With Google Voice, you can probably do the multiple-devices-per-phone-number now. I don't know if it works directly with SIP phones or not, though. Also consider that there's a carrier named Zer01 that is doing unlimited data only (they're an AT&T MVNO, and they're basically selling VOIP solutions for conventional handsets, and not offering any kind of conventional GSM voice. It would be interesting to see how these things might all come together.