I am also in grad school (biomedical engineering) and I'm in charge of the lab's computers (12-15 Windows desktops and laptops). But I bring my own laptop back and forth everyday -- it's a G4 ibook.
I could probably list reasons all night long, but here's a few:
The ibook never has the kind of problems that I have to fix daily on the windows machines. crashes, inexplicable slow-downs, lost internet connections, unrecognizable drives, virii, spyware, etc.
As a UNIX-based system a whole range of open-source software written by academics for academics is available to me. Reference managers, modeling and mathematical packages, file converters, voice recorders, etc. Often my colleagues ask about how to get something done, and I show them how I would do it on my Mac. Their next question is always, "Can I get that for Windows." And the answer is almost always, "No" or "Not without paying for it." If you do go with a Mac, PM me and I'll send you a list of the most useful free/cheap software for grad school. I've also seen chemical modelers, though I don't really use them.
The new elements of Tiger are also very useful for academics: specifically, Spotlight and native PDF markup.
Macs hold their value much better. I recently sold my 2 year old ibook G3 for $150 less than it cost me to buy a new ibook G4.
I don't think I'm exaggerating to say that for the same money you might get a pentium processor that would save you fractions of seconds for certain applications, but you'll be using an OS that will cost you HOURS of extra maintenance.
The Apple OS X has an elegance that makes it just more enjoyable to use. Maybe I just sound like a fanboy, but I think if anyone who uses both OS's for any length of time, will understand.
When someone asks me to fix their computer, my stock answer is, "Well, here's your problem...you have Windows installed." /ubbthreads/images/graemlins/smile.gif
It's not all roses; there some negatives too:
1. Commercial hardware and software is rarely discounted. You don't find the free-after-rebate deals that you can get for Windows.
2. Most peripherials will work just by plugging them in, but occasionally some devices from smaller companies won't have drivers. I have a laser pointer that works as a mouse without any special drivers, but I can't program it to have different actions with different programs.
3. If there is a specific software package that you absolutely have to use and that is Windows only, you have to think carefully about how to handle that. Virtual PC or borrowing a computer might work, but they are not optimal.
Anyway, for your needs I would strongly recommend an Apple machine. Specifically, it's hard to beat the value/performance/size combination of the 12" ibook. If you won't be carrying it around everyday, and want a little larger screen, go with the 15" powerbook.