From the Linux 2.6.9-rc4 changelog:

<schwab@suse.de> [PATCH] Properly recognize PowerMac7,3 	

Make the PowerMac7,3 no longer unknown.
Signed-off-by: Andreas Schwab <schwab@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

So that is nice already, let’s hope the Ubuntu guys will create a new installer soon with this patch applied. Maybe it will boot (it currently doesn’t at all).