[vhffm]

(Not Webdev, but Scientific Computing. Hope that's OK.)

Another vote for the OSX + VirtualBox/Linux combo on a MacbookAir here. Gives all the shininess of OSX, battery time of MBA, with the (package managed) flexibility of a Linux system. It's also seamless and transpartent when you mount your project folders, and run it headless with local port forwaring (ssh/http/...).

Also, if you need to do something crazy, you can just clone the VM, muck about, and then throw it away again.