ARM or x86, I don’t mind, but cheap and fanless is preferred, as is official Debian support.
The Fit-PC2 seems very promising, as it has room for a 2.5" hard drive and has 1000baseT ethernet. It has a GMA500 which has no open source driver, but presumably if you ran it headless it wouldn’t matter.
The Linutop also looks pretty good, though without the gigabit ethernet.
Anything else I’m not aware of?
Maybe the Sheevaplug?