Have you looked at the Sony PRS-505? Both my fiancee and I own one (I bought one for her as a present, and borrowed it so often that I ended up buying my own), and it satisfies most of what you're asking for. It's not technically "open", but there are
examples of people modifying it anyway, and it doesn't require any proprietary software to be installed on your PC. You plug it in, and it shows up as a drive, same as a USB key would. Put the books in the right folder and you're done.
It supports quite a few formats natively, and there's a wonderful
open-source program named calibre that will convert/format many other things to the best formats for it. Battery life is great, reading 3-4 hours a day it'll easily last over a week without charging. I'd at least give it a look, I don't really have any complaints about mine.
One thing to note is that Sony has put out a new model, the PRS-700, but it's generally considered to be inferior to the 505. They modified the screen to add touch capabilities and a backlight, and it's not nearly as nice to read on.