This seems to be a bit of a common problem from what I gather!
My installation on iPC 10.5.6 takes around 6 minutes to boot up to the desktop...
I've read of people getting this down to under 30 seconds!
It seems to hang for ages on the blue screen, just after a long pause (when viewed in verbose mode) for the IOKitWait line. I read in another forum somewhere that removing the unused kexts, especially the FireWire ones, can dramatically reduce boot-up times, but being relatively inexperienced in all this, I'm not too sure how to best proceed.
I've installed and run OnyX, which hasn't helped very much.
Can anyone offer any suggestions?