Seagate goflex drives (among others) are USB/fw bus powered external drives that can fit in a jeans pocket. USB is available now and TB to FW800 adapters are reportedly coming soon from Sonnet technologies. Keep it in your laptop bag and you'll always have up to 2 Tb at your disposal.
If this were true it would be sweet only because I switched everything from USB 2.0 to FW 800 as the TB drives are way too expensive for now, I am sure they will drop in price as more stuff comes to market.