Update 6/9/2011: I have put in a bug report (#629492) with the Debian developers to notify them of this issue.
It looks like there is a bug in the installer for Debian Lenny/PPC that causes the installation to improperly detect SMP configurations. I have a Quicksilver PowerMac G4 dual 800 – after I finished installing Debian I noticed that the server was only seeing one processor:
pengc99@baccus:~$ uname -r
Not to panic, the solution is actually rather simple. Just install the SMP kernel meta-package:
root@baccus:~# apt-get install linux-image-powerpc-smp
<!– lots of installing stuff here –!>
Reboot the server, and then check the kernel again:
root@baccus:~# uname -r