Yea, they won't default. Bond holders would be the first to get paid. Though I don't think they should flirt with a credit rating drop. While there are no immediate replacements for the dollar, it only strengthens the arguments of the oil giants (Iran/Russia/Brazil) and China to replace the dollar.
I agree they shouldn't flirt, but as far as I can see, their possible agreement won't give the rating agencies any solace. Not that I have much faith in them either.