my vote: iMac
I am a graduate student, and have used a laptop throughout my college years. A Mac laptop is an excellent option, but... If I were starting over today as an undergraduate, considering Apple's current product line... I would get an iMac instead, and have an iPhone or iPod Touch for carrying around campus.
I never took my laptop to the library because I was concerned about it being swiped if I accidentally dozed off (LOL, like that never happens in college). When I was at the student union or local Starbucks, I actually wanted to talk with other students rather than pecking away on my laptop keyboard. The only time when the portability of a laptop was of benefit is when I went home in the summer/winter breaks. However, I rarely needed to do school work over the breaks, so an iPhone and using family computers would have been sufficient. When you are going to class, you can have an iPhone or iPod Touch for staying in touch with your friends through email, texting, facebook, etc, and also for voice recording of lectures if you choose that.
Back in your dorm room, you have a computer with a large display, great for photography work (getting aperture?), renting and watching movies, and it can even be your television (get the El Gato TV tuner). Get some good speakers and think of it as an entertainment center when you aren't working. You can also put a huge hard drive in it for storing lots of digital media, etc.
As a graduate student, I'm now traveling more as a I transition from school to professional life (conferences, etc), doing more presentations, so a laptop for me at this time makes more sense.