Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Are you running Leopard? Many users running Leopard have noted a significant increase in boot time.
 
Perhaps you have a lot of startup programs. You can view a list of login items under User Accounts in System Preferences.
 
Look at the logs ...

I looked at mine and realized that I needed to rerun something because the Quicktime update had thrown something out of whack.

Hopefully after a boot cache update (darn throttling respawn again) and a permission repair, thing will settle back down.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.