The My Book Thunderbolt Duo will house a pair of hard drives that can be configured as a RAID 0, RAID 1 or as a JBOD, and will be pre-formatted for the Mac using HFS + Journaled, which means that it will be compatible with Time Machine right out of the box. It will also sport two Thunderbolt ports to allow for daisy chaining, although it should be noted that the more devices you daisy chain it with, performance will undoubtedly be affected.
Similar to other hard drive manufacturers who offering Thunderbolt devices, the My Book Thunderbolt Duo will not be shipped with a Thunderbolt cable, which means that if you were to pick one up for yourself, you will need to spend an additional $49 for a Thunderbolt cable which is available from Apple. [Product page]