If the Japanese story is one to go by, and so far it is, a downgrade of US debt by rating agencies will have hardly any impact. I don’t see what information the rating agencies have that everyone else doesn’t already have and despite tough economic times, I don’t see the US defaulting anytime soon.
Prediction: If the US debt is downgraded in the next 6 months, yields won’t increase by more than 0.2% 6 months after the downgrade. In other words, there might be a small, temporary uptick, but within 6 months yields will have returned to below 0.2% above the day before downgrade.
[update: The US has now been downgraded from AAA to AA+ on 5 August 2011. 10 year yields are 2.56% after a significant drop over the last few weeks from already low levels. We’ll have to see whether come 5 February 2012 10 year US yields are above or below 2.76%]