A friend recommended Sidenote, and it turned out to be exactly what I was looking for -- like the Quicksilver Shelf (which I'm using religiously nowadays) it sits in drawer on the side of your desktop, can be pulled open momentarily (either with the mouse or a hotkey) and then slides right back out of view when you're done. Just like TextEdit, it allows for a nice variety of text formatting, and unlike TextEdit, it saves in a repository rather than a file. I only use one note so far, but there's functionality for multiple notes in there as well.
We last mentioned Sidenote way back in 2005, and since then it's been upgraded to 1.7.3, and streamlined a few already streamlined features. Very nice and easy app -- for the purpose, it was exactly what I needed. It's available as donationware from developer Pierre Chatel.