9/10 live laptop performers prefer Apple.
Apple also make the best consumer laptops in the world. From battery life to dimensions to fan noise to weight to maintenance to operating system, it's all a little superior to the alternatives.
The iBook comes with pretty good specs these days, and is upto live performance pressures. The best laptop set i caught last year was Kaffe Matthews on an iBook. Powerbooks are dearer, but worth it if you got your bling on.
That said, you can toss a cheap Dell running Ubuntu around to your heart's content. And i've seen numerous performers run on linux. I wouldn't discount this option if you're quite computer literate and comfortable in the shell. Sony produce ok PC laptops. Avoid Samsung & Compaq, imo.