Folkert asks, not without reason, whether my debian-installer support for NBD is available for testing. The answer is: yes, it is!
At DebConf11 in Banja Luka, I sat down with Otavio, who helped me get the necessary modules uploaded in the correct way. As such, you can now install debian to an NBD device with a regular debian-installer build—although for now, you'll probably still need a daily build to get it to work[1].
To test, you should do the following:
And that's it! Everything else should Just Work(tm).
Well, almost. There's some loose ends that I need to fix, which I haven't had the time to look into yet. Hopefully that won't take too long.
[1] It's possible that cdimage builds will work too, but I haven't tested that.