This doesn't mean the MBA is a bad computer at all -- for most tasks, it'll work just fine. But complicated 3D graphics (both Halo and Unreal Tournament 2004 were tested) will be almost unplayable in games, and there will be a substantial wait, sometimes up to minutes more, for certain processor intensive tasks. If speed is a high priority (at least higher than portability), the MBA isn't for you.
I would like to see how the MacBook compares, though. It's not surprising that the MBP is a fast machine, but where does the MBA line up compared to the cheaper model?