The GoFlex Slim weighs 0.356lbs (160g) and measures 4.91 x 3.07 x 0.354in (124 x 78 x 9mm). That's pretty impressive when you consider most 2.5-inch notebook hard drives are 9.5mm thick without an enclosure. Being only a bit larger than today's smartphones, the drive should be easy enough to slip into a jacket pocket.
Seagate has opted for a spindle speed of 7200RPM instead of 5400RPM and the GoFlex Slim uses USB 3.0 for both data and power. We haven't seen any official throughput figures, but Engadget reports that the drive reached upwards of 40MB/s during an in-house transfer test, which isn't too bad all things considered.
The GoFlex Slim is currently available online for $100 and comes with backup and encryption software as well as a three-year warranty. Although it's preloaded with an NTFS driver for Macs, you'll have to reformat the drive to HFS+ to use it with Time Machine. An HFS+ version will hit Apple stores sometime next month.