The real story of course is if GM's debt is downgraded to junk. I believe it's only one notch above right now. That is the event that would open the floodgates of pain on the whole market, not just GM. The automobile sector is the largest borrower in the corporate debt markets, and GM is the largest of them. If their bonds went to junk the amount of forced selling would overwhelm the corporate debt market (the charter of many income funds does not allow them to hold junk). Corporate spreads in general are the tightest they've been in two decades, so there is a lot of room for things to come off. The corporate debt market is very very firm right now and if it did manage to stabilize without major damage, GM is still screwed because they have so much debt that needs to be rolled over next year.
You can buy exchange traded GM debt that after today is yielding about 8.5%. I'd own that way before I'd buy the common for it's yield. Wouldn't touch it with a ten foot pole right now though. As a frame of reference, Delphi Automotive has one class of debt that is one notch below investment grade and it's yielding about 11%. Tread carefully.