I use an HP/Compaq NC6000, $300 on eBay. Make sure you get the widescreen (1440w) version with integrated Bluetooth, and expand it to 1GB RAM. It runs everythin

I use an HP/Compaq NC6000, $300 on eBay. Make sure you get the widescreen (1440w) version with integrated Bluetooth, and expand it to 1GB RAM. It runs everything I need for interactive development, including hogs like Photoshop, Flash, and the Eclipse-based Flex Builder 3; the Bluetooth means I can get internet access through my smartphone; it's fairly light and has a real-world battery life of three hours under steady use; its battery is swappable, so you can carry a spare; and you can afford to lose it, which is a very distinct possibility if you're carrying it around a bustling city all day. (I live in Manhattan.) Even the best backup plan and warranty won't cover the "oh FUCK ME" moment when you realize your $3000 laptop just got lost, dropped, or stolen.