ragingquietly found it! The ad is here and the song is by Nick Drake.
Second, another ad: Lincoln supposedly lists all their songs on their website, along with download links for the songs featured in said ads, but there's a new one...for the 2011 MKS, I believe (anyway I know it was the Lincoln MKS, and it wasn't any of the 2010 spots), and it's not on the website and I can't find it on Youtube. It's also not Shiny Toy Guns' "Burnin' For You" on the EcoBoost commercial; I'm 99% certain it was for a brand-new 2011 MKS. And I don't remember anything about the song because I haven't seen the ad in a while. >_<
Finally: I've heard this song a couple times on the radio but no one at the station ever said the name of the song or singer, and I thought I remembered enough of the lyrics to Google them, but nothing came up that looked right. It's a female singer, I think there's a steel guitar in the background of all or part of the song, and the lyrics I can remember go something like "I'm an acrobat, let me show you what these legs can do" and there's something about bending instead of breaking, I think...? I suspect it's by a fairly obscure singer since Googling the lyrics didn't help me any and it was on a college radio station. Sound familiar to anyone?