See the comments at the bottom: thus far GMAC has been the profitable part of the equation.  Although I dabble in bankrutcy work, these issues are beyond me, except to say the obvious that creditors in bankruptcy might take as much or more of bath because of the automatic stay than cramdowns and other Chap 11 provisions.  Remember GM is the largest health care provider in the world and I beleive has the largest pension system after the U.S Gov't - (could be wrong on the latter assertion.) How a long Chapter 11 work-out would play out for the hedge funds and other creditors, bondholders, etc I don't think anyone can know. This is complex along the lines but not the scale of PO and Global Warming.

[Incidentally, Auto lease are increasingly complicated as they now include built-in service agreements and other consumer unfriendly provisions that offer substantial margins to the dealer and automaker over-time. Many (most?) lease are underwater for a time with consumers as the vehicle depreciates significantly before payments catch up.]