I've been using a previous generation MacBook Air (1.86GHz Core 2 Duo, 2GB RAM, 128GB SSD) for web dev work for 18 months or so. It's great for everything _except_ browser testing in VMWare. The 2GB of RAM just doesn't cut it when I need to fire up Windows to test in IE or, god forbid, the BlackBerry simulator for email testing.
Because all three are still very buggy on Wine/WineBottler, and there's no way to tell the difference between Wine/WineBottler/IE issues and bugs in your application.
More and more, I find myself renting EC2 instances for small projects instead of using VMWare. The new micro instances are super cheap at .02 and .03 per hour for Linux and Windows, respectively.
Granted, I still use VMWare/Parallels periodically, but my reliance has started to wane in the past 12 months.