Hello Michel,
Really? Can't you just partition your hard disk in two for 10.5? I'm
not saying you should take the burden of testing with both, but saying
you can't install 10.5 alongside 10.6 seems dubious to me:
1. Use Disk Utility to resize your current partition and create a new
one.
2. Boot using your 10.5 installer DVD and install on the new
partition.
3. Press option at startup to see a boot menu; open the Startup
preference pane to set the default boot partition (or use "bless" on
the command line).
IIRC Walter mentioned in the NG that his mac box is running as a headless
box in the basement somewhere so boot switching might be annoying.
