So a common theme across various news sources is that failure to agree an increase in the US debt ceiling could be at the very least a shock to the global financial system. There seems to be a consensus that shares will probably drop in the region of 20% even if there is only a short interruption.
The BBC have a summary here.
A simple guide to debt ceiling, and what the 14th amendment has to do with it - BBC News
Any of our educated members care to elucidate, particularly ones who might have worked in related fields and seen the effects of these standoffs before?