Yup, that is the sled for the X-serve drives. Apple does not sell it separately but on Ebay it looks like they go for $155. Apple locks out all other drives since only their "blessed" drives sold in this sled can go into the server unless you buy them from someone on Ebay.

That gold coated wire in the bottom of the pic lights the LEDs on the sled handle and the power and data wires which are hooked to the drive wire the compact connector which is on the right had end which connects up making it hot swappable.

I might buy a sled and put a 2TB drive into my last bay to see how it works. I'm sure it would work fine.