Hi Josef,
This may be 'good news/bad news' I had bubba working fine with two separate Macs and Time Machine with Leopard 10.5.1.
I used the "defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes -bool yes" hack as an administrator at the terminal in the Macs.
I then used [Apple]+[k] in the finder to connect to a write-share on bubba - I used "smb://bubba.local/home/tim/macbup1" (macbup1 is a folder in my home directory). When I set up time machine on the Macs the shared folder was visible as a disk. The macs have been reliably backed up now for a month or so, generating a few hundred MB a day each - The first back up was very slow!
Now the "bad" news - This is definitely *NOT* supported by Apple! Several people recommend against using this hack. The "worse" news is that since I upgraded the macs to Leopard 10.5.2 shared disks are not showing up in "Choose Backup Disks" - I suspect that Apple have disabled this hack in the 10.5.2 upgrade - You may be able to get it to work in 10.5.1.
I would recommend that you use rsync to back up the Mac onto bubba - I use this routinely to back up one bubba onto another (see other threads in this forum). I have just quickly logged on to bubba with ssh from one of the Macs and tried "rsync -av -e ssh tim@192.168.0.2:/users/tim/documents /home/tim/test1" where 192.168.0.2 was the address of one of the Macs - and it worked. You will need to allow "Remote Login" from Sharing in System Preferences from the Mac before you try this.
There is a good article here
http://www.bombich.com/mactips/rsync.html by Mike Bombich on rsync on the Mac.
Regards.
Tim