I'm just finishing off a PhD in history and there's nothing more involved (computer wise anyway) than large word files and pdfs with large images, pretty much any computer can handle it.
Complex theories are being tested (hopefully) and there's a lot of data but it's not quantitative, so nothing that I have to use a computer for, and I'd imagine my situation is fairly typical in the humanities.
I faced the same question when the MBA came out (was then using a MBP) and in the end decided it wasn't for me - now I use a Thinkpad. I don't know if the reasons for my choice are relevant to the OPs use, but here they are:
1. No security slot on the MBA. So if you are in a research library, you can't secure it to the desk while you go for a coffee break (of which there will be a lot since you will likely spend the whole day there). Taking the computer with you to the cafe is a pain - it's another thing that has to be searched on entry and exit, and as proper bags aren't allowed in, you will have to somehow carry a cup, your laptop, documents and food with only two hands.
2. No keyboard shortcut for footnotes on the Mac version of Word, unlike on the PC. If you use footnotes really frequently, this rapidly gets annoying. even on the MBP, there was also a lag between typing and the letters appearing on the screen, quite slight but still a bit annoying. There may be better word processors out there for OS X, but..
3. If the people you have to send documents to can't read them, or they come out badly, it IS YOUR FAULT even if they are running Word 6 on Windows 95. Seeing as you will almost always be asking for something when you send a document out (journal or conference paper, feedback from tutor,funding application) you can't tell them to buy a mac
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. In my case, seeing as I have to deal with people running Word on PCs, it's easier just using it myself rather than trying to remember which fonts don't print on Professor X's computer.
4. Kinda the same as Point 3, but for things like bibliography software - the the most often used package in your department/subject area is PC only (Reference Manager etc.) there might be problems importing and exporting to mac comaptible software. Not an issue for me, but it may be for the OP.
Anyway, for these reasons I got my Thinkpad - I don't want to turn this into
a mac vs pc thread, but just to share my experiences.
btw, I hardly ever see people with the MBA in the British Library - I'd say it's roughly evenly split between PCs, Macs, and netbooks, with netbooks probably having the largest share by a slight amount, fwiw.