in reply to Perl on a Macintosh
There are so many personal choices and preferences that it's really difficult to make any firm recommendations.
I tend to buy Windows boxes, because they're around. Then I install Linux, if it will take. If not, I reinstall the Windows and live with it.
Oddly, I do most of my development on the Windows computers, simply because my tools are mostly useful on Windows. Even ActiveState's Komodo, which runs on Linux, has features missing that appear in Windows.
I have heard many good things about the new Mac's, but I've never used one.
Probably best bet would be to go around and test drive the units that look promising, to see what seems to fit.
BTW, Dell sometimes has laptops available for around $400. You'd probably need to get more memory, but it's something else to think about.