I asked for it a few blogs ago and now I've got it: the first pocket-size external hard drive that features an eSATA connection, the eSATA OWC Mercury On-The-Go. eSATA is the external interface for SATA, currently the most popular interface for internal hard drives.
A while ago, OWC introduced the world's largest small external had drive and has now become the first vendor to put eSATA on a compact external hard drive. The eSATA OWC Mercury On-The-Go also supports USB 2.0 and features a 320GB internal hard drive from Hitachi that spins at 7200rpm (as opposed to the 5400rpms in most external hard drives of this physical size). It is also the largest in capacity among high-speed, compact external hard drives.
The drive is bus-powered when used with the USB 2.0 connection and requires the included adapter for the eSATA connection. This is because by nature, the eSATA connection doesn't (yet) support drawing juice from the computer to feed the external drive.
The eSATA OWC Mercury costs $250. You can get a USB 2.0-only version for less, or the triple FireWire 800, FireWire 400, and USB2.0 version if you're willing to pay little more money.