I just wanted to add a comment for anyone who might be considering purchasing a new car, and this is just based on what the person who sold me my car said back in November (this is at Saturn).
They were offerring 0% on the model I bought, and when we were discussing down payments, he said that usually they don't ask for a down payment for anyone who has a credit score over 700 (mine was high 600's at the time), and I think he said to qualify for the financing you needed something like 620. So anyone below 700 and above 620, they were only asking a down of at least enough to cover taxes and fees (about $1000). (I'm not sure what happens for people below 620 though, I just assumed that they didn't qualify for the 0%, but not sure about a higher percentage.)
Very insightful thread, thanks to everyone who commented!