Of course, if you bought GM Bonds Obama would give you the ultimate haircut... a 100% loss on you bond investment so he could move the residual capital to the UAW pension fund... more like a decapitation than a haircut. Doesn't the constitution say something about the need for due process before you are deprived of property by the government... How's that working for you?
Sovereign debt is an entirely different animal. Whereas GM and other corporate bond issuers must pay interest and ultimately principal by earning profits, sovereign debt is paid off (IF it's paid off) via taxation of citizens. And, in the Greek situation, bondholders (well, the sane ones at any rate) understand that there is no chance at all that they'll be repaid in full. Both bondholders and would-be bond guarantors -- whether EMU, ECB, or individual members of EMU -- are only arguing over the level of haircut that will eventuate. No one is arguing for full repayment.