I am one of the biggest MBA fans on these forums, but I must provide a dissenting opinion here (pun intended).
I recently finished a PhD in Industrial and Organizational Psychology and used an MBA for about a year. Now I was trained in quantitative research using statistics packages like SPSS, LISREL, Mplus, R, MultiLog, ggum, etc. and I must say that the MBA was not suited for this type of work.
The main problem was that most of the statistics packages only work in windows. The MBA is SEVERELY limited by the 2gb of ram when it comes to running virtual machines.
example, real-life scenario:
Problem: You are in an item response theory class and are asked to run item characteristic curves using ggum for an experimental biodata measure for use in a large manufacturing organization.
Solution: After trying to use my MBA to run a virtual windows 7 machine, which itself needed to run an XP VM (since ggum only works on XP), the macbook Air quite literally crashed under the pressure. I had to look over my neighbor's shoulder for the rest of class.
The next day I brought my 2.66ghz C2D with SSD to the class, hoping the same situation would not happen again. We were asked to run the same analysis... so I started up the Windows 7 VM, running the XP VM, and the same run took only 32 seconds, the fastest run in the class by 15 seconds.
This long story isn't meant to dissuade you from the MBA, but just be wary of the limitations of the machine. Perhaps it will work for you perfectly, but it didn't for me. I ended up getting rid of the MBA and I couldn't be happier with my decision to keep the MBP... now if a new MBA were to be released, I would have to think long and hard about getting it!
Anyway, good luck on your PhD!... it's a long road, but it's certainly worth it!