I just bought a Seagate FreeAgent GoFlex 1TB and I love it. I tried another external before that had a USB Y-cable and didn't like it much. It had to use both USB ports to provide enough juice which would've been difficult to pull off on the MBA.
On the GoFlex, it is really quiet and can power itself on its own cable. The only con I can think of is that it uses a proprietary connector. The plus side though, is that the connector can be switched out for other "accessory" plugs like FireWire, USB3.0, eSata, TV dock, and "nas" dock. I'm assuming that if ThunderBolt becomes immensely successful, Seagate might go ahead and release a TB connector accessory for it.
Mine came with the USB 3.0 plug (not the 2.0 one) and I got it cheap for $79. It was an open/returned item and the neatest thing was that it came with a ripped movie that is in a language I am unfamiliar with!