Using something like http://www.debian.org/ports/kfreebsd-gnu/ you'd be stuck with the same
driver support as the original FreeBSD kernel, as that is basically what it is. A FreeBSD kernel, with a
GNU userland and libc. It's going to be the first non Linux based Debian release, being fully
supported from Debian 6.0 (Squeeze).
As for the NetBSD port of Debian it's not maintained, so it'd be a very bad choice.