1) Has anyone already done this? If so, please point the way to them.
2) Do you think it would it be hard to do? What obstacles do you foresee?
3) Which SIP client would you use?
4) What cost ballpark would I be in to have one of the experts here at XDA do it as a paid project?
5) Do you think you could roam between data services in mid-call?
6) Would you have to stay within one line of devices?