I just did a local 48 hour film fest with my 5D mark II and my new MBA 11". I usually do all of my work on my 8-Core Mac Pro, but I wanted to see what the MBA could do. After every shoot we had a runner back to the house to dump and convert the 1080p files from the camera to 720p ProRes(1080 was pointless for this particular project) on the Mac Pro, and then we did our first cut in FCP(13 minutes long) on the MBA as we had to be mobile. it was flawless.
It's perfectly capable of being a quick mobile video editor. You just need to be weary that rendering complex effects/mattes will take a bit longer, but if you're just cutting something together it's not like you'll need to render those types of things out all the time.
