I've gone through a month of college without a computer (just using labs when I needed one) to see if I wanted a laptop or desktop and hoping to get lucky with a new hardware revision.
I went with a desktop because I know I won't be able to pay attention in class with an iBook or PowerBook, and my roommate has taken his laptop out of his dorm room only twice. Sure, on several occasions I've told myself it might be nice to have a laptop, but on second thought those situations have always been luxury not necessity, and often they weren't as practical as I originally thought. I could take a laptop out onto the lawn and work in awesome sunny weather... but in reality I'm more likely to fall asleep than finish that paper. In bad weather, I'll be working in my room anyway. With this in mind, I find laptops still too fragile and expensive for their specs.
Turns out the new 20" iMac was perfect for my needs. I was going to get an iSight anyway to video chat with my family back home. I didn't have a monitor and would have gotten an LCD because of the limited space on my desk (and now I don't even have to worry about a tower!). The screen is awesome; it'll be a TV replacement and my roommate and I will be able to watch some awesome DVDs on it. I'm running an 1/8" extension cable from the computer to my headphones so I can lie in bed and listen to music yet still be able to skip/play/pause.
Like other people have said, hardware will change rapidly in the next four years. (Intel...?) Once you figure out how professional you want to be in the future, you'll be able to get a machine that better fits your needs. You can get overkill now and still have to upgrade it later, or get something that fits and get a whole new, better machine later. You can't plan ahead 4 years in computers.
The iMac is a 1.9-2.1ghz G5! That's over 4 times as fast as my previous 533mhz G4. It's an awesome processor, and I'm going to love it. It'll definitely last 4 years.
So if you're in the same situation as me, get an iMac.
