Private investors are supposed to take a "voluntary" 50% haircut on Greek sovereign debt. The reason why EU negotiators worked so hard to get it called voluntary is that they don't want the default to trigger payment clauses in CDS contracts.
Why they so strenuously object to this is not fully clear (at least to me). Maybe they feel like if the CDS don't pay out, it's not really a default? Maybe they think they are being clever and "punishing evil speculators"?
But it's not really that simple.
First, the ability to buy insurance puts more people into the Greek (and Italian and Spanish) sovereign debt markets than would otherwise be there. At the margin, invalidating these insurance policies will drive people out of the very markets the EU is begging people to enter.
Second, if I ran a bank that held Greek debt that was hedged via CDS, I would fight like hell against accepting the haircut. After all it's voluntary, right? I'd wait around until a haircut that would trigger my insurance payment came on the horizon. And, if I somehow got strong-armed into taking the "voluntary" 50% reduction, I'd litigate and fight like hell to force the insurance to be paid to me anyway.
However, I guess I wouldn't worry too much about the CDS not triggering yet though. This deal, as a best case scenario (i.e. everyone accepts the voluntary haircut and Greece hits all its promised revenue and deficit targets for the next decade) reduces the Greek Debt/GDP ratio from 180% to 120% in 10 years! Why doesn't a 50% haircut cut the debt ratio by at least 50% (more if you think the Greek economy will expand at all in the next 10 years)? Because at this point a lot of the debt is payable to the ECB, IMF and other "official" creditors who are not taking a seat in the barber's chair.