I've had my MBP for a little over a year now. It used to boot up lightenign quick, to the envy of my Windows friends. Not so much anymore.
What can I do to speed up the boot time on my OS X 10.5 MBP?
Get rid of any accumulated cruft you don't need, e.g.
Startup Items (Accounts -> Login Items prefs)
Background services such as web server, universal access, bluetooth etc.
Extra preference panes
Make sure you have enough (at least 10%) free space on your HD
Don't reboot? The only time I actually reboot my Mac laptops is after installing system updates. The rest of the time, I just let them sleep. This has worked well for years. The only time that this isn't an option is if won't be plugged in for days, since it does use a minimal amount of power.
I always do some maintenance every month, which usually involves clearing a few caches etc.
I feel as though it makes things faster, so it may help you too.