I guess it depends on where you're traveling. If traveling where there is ample free wifi (in the US), then I'd use dropbox or a similar service and not carry an external hard drive. If traveling where there isn't ample free wifi (Europe) then I'd use a Seagate Goflex drive. It's not the lightest you can get but it works well and isn't the priciest either. They nice thing about the GoFlex drives is that the connector is interchangeable, so you can use a Firewire 800 connector, or replace that with a USB 3.0 connector, or, soon, a Thunderbolt connector. Since the drive isn't built to take full advantage of the Thunderbolt connector, it'll perform like a Firewire 800 drive (which ain't too shabby and is the only way you'll get that performance directly on a newer Macbook Air).
And I'm either not understanding Bob or disagreeing with him - current SSDs are faster than 'normal' platter drives (often by quite a bit), consume less power, and, theoretically, should last longer. They're certainly more durable in the short term. There are external SSD drives, which should be lighter than their platter cousins, but they're still much more expensive than platters.
Depending on how soon you need it and what year Macbook Air you have, a few companies are coming out with external SSD drives with a Thunderbolt port. These should be blazing fast, fairly light, and exceptionally expensive. One company, OCZ, is supposed to come out with one up to 1TB.
Elgato (of eyeTV fame) has an external Thunderbolt SSD drive. It's $800 for a 240GB drive and weighs 9.5 oz. (240 grams). Reported to be available this month.