It's unlikely that the US will ever really default in the near future. Why ? Well, the global economy is becoming more and more linked as time passes. A US default will trigger a collapse in the larger global financial markets. Do US creditors want this ? Nope. They'll just keep extending deadlines but of course this charade can only last for a certain amount of time. Personally, I think the collapse will not come from a US default but due to this bubble over extending itself and eventually popping in a Lehman'esque style.

An example of this is how last year India (a developing country, although with a rather large GDP) donated money to the European Union in the form of an economic stimulus, basically to prevent a default. The reason was simple, an unstable european market hurts growth prospects worldwide. There's a saying in India "When Wall Street sneezes the Indian markets catch a cold". An increasingly linked global economy puts economic stability at #1 on everyone's agenda. Of course, this is great for the US because they can wage wars, etc. etc. while taking on unlimited debt and no one wants to lift a finger (militarily, they can't - economically, they can't either because of the repercussions). Of course the bubble will pop at some point. We don't know when, though.