Cost of Governmental Sub Prime

I've asked before if our government was upside-down in debt and concluded it was not. But what are the costs of that even being in question? I recently came across a site that gave the US government a FICO score of 620 (which puts it in the sub prime category). Just like a person with a lower credit score, this could increase the interest rate at which the government has to borrow. However, it doesn't look like that is currently the case. The US government has a triple-A rating, the best there is. As long as our borrowing starts to decrease, or at least increase less quickly, I won't be too worried about our government it's reputation as a reliable borrower, but it is on my list of things to watch out for.