I've no problem at all imagining a Greek default, the question is whether that means it has to leave the Euro...
Although Greece is a guarantor of the currency, it's a tiny % of the Euro land economy, so I do not see any reason why it must be kicked out and I don't believe it will choose to leave this side of an election.
Part of me sees Greece not as real sovereign debt but more like a corporate in a given currency zone, a default in $ doesn't mean you're not in the USA any more.
My feeling is that we wil see a "technical default", ie bond holders being obliged to lengthen the maturity of their holdings.
That forms part a "least bad" situation, the regulators could choose to treat the default as not affecting the underlying value of the holdings keeping them solvent.
Also, that would cause the value of Greek and other troubled Euro states debt to increase, that should happen because once the market knows the boundary of badness people will start bargain hunting.
I believe someone is going to make Soros level money on buying the pathetically cheap Greek 2 year bonds which will become (perehaps) 5 year bonds. Nice gain from both the price going up and the established ooupon.
...or they will be utterly destroyed by this gamble, you choose.