Most of this perceived difference is just you're getting accustomed to the boot speed of OS X, and resetting your standards by it. The rest could probably be accounted for by things you've done. Dashboard widgets you might have added need to be loaded, icons on the desktop, any programs that lodge themselves into the application bar, services you might have enabled... They all take time to process.
I bought a black apple macbook in august 2006.
When i first booted the macbook i was pleasantly suprised at how fast Mac osX booted up.
Now (March 07), I have noticed that the booting seems to take much longer than before, but the overall performance of my Macbook is unchanged. I have stayed on the ball with all firmware updates.