Re: Upgrade to Leopard...
Sure, I understand...
Do what Yadj says and drag your Logic preference file (plist)to your desktop - don't trash it for now.
When doing a major OS upgrade e.g. 10.4 to 10.5 on any machine intended for Pro Use (i.e. you make your living off it) it's a better idea to clone, format and then reinstall all from scratch, rather than installing on top of. Having said that I don't always follow my own advice - and I've been fine.
Let us know if moving the prefs (which essentially allows Logic to recreate it's global pref's works.
How far exactly does Logic get in the startup process. Do you see any splash screens? Any drivers loading?
Lastly, and very importantly, have you updated any / all MIDI drivers for any ext hardware equipment you have - so it's compatible with Leopard?
Can you list all hardware you are using? And try disconnecting everything, restarting and then open Logic.