The installer runs a system check before it installs iLife (as it did on my G4 laptop) to see if your system meets the minimum requirements (a dual 2GHz G5 PowerMac or a 1.9GHz iMac). If you don't meet them, it won't install iMovie, and you're stuck with the old (but oddly, better featured) iMovie HD.
That is, unless you own on the of the original 1.8GHz single processor G5s from 2003. I installed iLife '08 last week, just for the upgrades to Garageband and iPhoto, but miraculously, iMovie '08 installed as well.
And it works rather well. Playback is smooth enough for me, scrubbing through frames works well, as do the transitions and titles. And I didn't even need to perform any of the dodgy hacks that have been going round to make it work on my system.
Since my system was only around for a few months before being discontinued back in 2003, maybe they've forgotten to put it in the 'exclude' list, who knows?