Final Cut Pro X on MacBook Air
I have a 13-inch MacBook Air with a 1.86GHz Intel Core 2 Duo and 4GB of RAM.

In my experience, it's just too slow to use productively. Screen size is also a concern once you start really using it. I haven't tried it in clamshell mode when hooked up to a Cinema Display, though.

It seems like you'd want a faster CPU as most of the rendering happens in the background, which can really slow the whole machine down when you're trying to edit.

It would be interesting to hear from someone with a new i5 or i7 MacBook Air, but either way, you'd want a larger screen if you were planning on using it without an external display.