Buying a Macbook only to put Ubuntu on it seems like a bit of a waste. You get Unix out of the box, and the battery performance is stellar. The gesture support is awesome, but I don't know if the drivers available for Ubuntu will help you maximize that. There's also quite a lot of very good software available for OS X that's not available for Ubuntu (Alfred, Tower, F.lux, and Dash come to mind).
This isn't to say that Macs are the superior choice, but from what I can tell you'd be throwing away a lot of the above by switching to Ubuntu, and those are a big part of the value proposition of the Macbook. If you're going to shell out for the Macbook, just use OS X.