In principle it is possible. I have a running Kubuntu (Ubuntu with a single KDE meta-package added which overrides GNOME) environment installed from a FAI Debian install server (which at least at that time was the most simple way to do it). But this is quite old (Kubuntu 5.04 with FAI 2.8.4 so I can't tell something concerning the recently split up fai packages in connection with Kubuntu 5.10 (or even 6.04).
Anyway, the main idea was (if I remember correctly) to remove the original Debian debootstrap package on the install server and to replace it with the Ubuntu debootstrap package to be able to create an Ubuntu-nfsroot environment on the install server. A big advantage is to have an install server of same architecture (e.g. i386) as all the client machines to be installed. Otherwise you have to setup an interim install server of the clients' architecture on which you only create the nfsroot environment which then needs to be copied to the 'real' install server (e.g. we have an i386 (Xeon) install server but all our clients are x86_64 (Athlon64) machines so I had to prepare the nfsroot on one of the clients first and then copied it to the i386 server). With the next Ubuntu release end of april I will upgrade my installation and most probably will write down some more helpful notes on the installation process... :-) Andreas