Get one of the ThinkPads -- I bought a replacement ThinkPad battery for my own T400 for like $30 on eBay, and it has been doing great.

I would personally lean toward something 14-inch, but a T500 will be a very competent machine, even if it is bigger than I'd buy. (And who knows what sizes the T60/61p are.

As mentioned above, I'd stay away from the 61p just because the BGA holding the GPU in place is eventually going to fail.