My understanding was that there were legal and technical barriers to prioritizing payments. I wouldn't count on this happening, if it requires new law to be passed or ancient computer systems to be overhauled within the next week or so.
I wonder if there are "emergency powers" that the Executive branch can invoke (kind of like - "we are under attack" type powers) that would allow them to go around Congress and just ignore the debt limit.
I can't imagine the US defaulting on it's debts....but then again....you never know.
http://www.nytimes.com/2013/10/08/opinion/obamas-options.htm...
The fallout would be epic.