I was looking at both types of computers as well: I chose the MBP (pre-penryn). I made the switch to Mac but also have VMWare fusion to run XP VM's for my java developing (work) and Vista (from my boot camp partition).
I feel like I definitely made the right decision--I have a hassle-free computer (OS X is amazing) and the option to run windows when I need to (IBM Rational Java development for work).
The T61 is one solid computer--but having to muck around with Linux is a pain and Windows Vista/XP, though decent, just aren't as versatile as OS X (of course this is based on my experience). Plus, you get to work in an environment with the option to use a Unix Terminal (no need to mention MS's Unix Terminal here...). The experience you'll get learning and using Unix commands for your development is invaluable.