As sethg pointed out, there's a difference between privete companies and sovereign countries.
When you lend money to a company and that company can't pay you back, bankruptcy law gets in the game. But when you lend money to a state, you know that there's a calculated risk that they won't be able to pay you and you won't be able to do anything to force them to pay your money back. You protect yourself by adjusting the interest rate. But if the risk is just too high and you lend anyway, it's up to you to take that bet...
If that country can't afford their debt, they can try to renegotiate with you, so you can get paid at least something. If it were a private company and the creditor does not accept the new terms, they could ask for a bankruptcy. As you can't do that with a country, if you reject the new terms, then the sovereign state can say "screw you" and never pay you anything. You had your chance to get something, and you lost it.
Some vulture funds buy this kind of bonds, and then try to reclaim the money... As I said earlier... good luck with that.
Even the U.S. Supreme Court and the US Government agrees with this [1].
That's why they are now trying this kind of weird actions, in some place where they managed to make a judge put his signature to their service... But it's mostly a media show.
[1] Supreme Court Rejects Elliott's Argentina Appeal http://www.finalternatives.com/node/20864
And some more extra info, even mentioning US Administration's support to Argentina's claim: http://marceloballve.wordpress.com/2012/06/27/who-pays-when-...
