I'm running 10.5 on my 1.42ghz PPC Mini. It works fine. More sluggish than Tiger, but I only have 512mb RAM in it.
Touchscreen is busted, so can't say if Lilli drivers work. GPSDX doesn't do very well, though. I was using it on a Royaltek Sapphire GPS with USB to serial bridge feeding Roadnav, Kismet and Google Earth. On Tiger, no problems. Leopard seems to continually lose track of the GPS and roadnav keeps reporting no GPS detected.
Switching it over to just the serial address fixes the problem but of course, you can't share the GPS output anymore.