The MB has a removable hard drive. And unlike theMBP changing it will not void your warranty. The HDD is located right next to the RAM in the battery compartment.
I ordered my macbook, and it had the 120gig hard drive...and i only had about 96gigs of usuable space.
so far, in the 2 weeks ive had my mac, i have around 74gigs left (damn you isight camera, photobooth, and imovie for being so addicting!) , and im thinking of getting a bigger hard drive in a little bit.
If you're making a lot of video, as nice as it is to have everything internal, you may consider the alternative of a 500-750 gig USB2 / FW 3.5" external. It'll take you a lot farther than any internal drive will, and then you will be free from worrying about your movie-making habit.
Although, the other thing you can consider is that you can get an external case for the existing 120GB drive for ~$25US and then use *that* as an external, in addition to whatever you replace it with....
I like how the MacBook is very easy to upgrade, and how it does not ruin the warranty. I'm guessing if your really into the PhotoBooth and iMovie a bigger HD would be good.
Man, I wonder what is going to happen to me when I finally get my MacBook. Haha.