At first glance it's clear that the new Western Digital Velociraptor is not your typical 3.5" hard drive. In fact, it's not really a 3.5" drive at all.
While the WD Velociraptor does have a 3.5" form factor, the actual mechanics are housed in a 2.5" HDA drive with a 15mm z-height, similar to standard notebook hard drives--only taller. The drive itself is mounted in a what is essentially a large heatsink, that WD has dubbed the IcePAK. The IcePAK is what allows the WD Velociraptor to be mounted in any standard 3.5" drive bay and, of course, it also helps keep the drive cool.
As you probably surmised, the WD Velociraptor is targeted at performance enthusiasts, workstations, and low-end servers. Its 300GB capacity comes from a pair of 150GB platters spinning at 10K RPM within the drive mechanism. Four heads access the platters, and because the platters spin so quickly, and are physically smaller than standard 3.5" drives, the Velociraptor is able to access and transfer data much more quickly than competing offerings.
A visual comparison between the WD Velociraptor and a previous-gen 150GB Raptor reveals almost no similarities, other than their coloring. And be aware, because the WD Velociraptor is not a standard 3.5" SATA drive, its connectors are not in the same position as other drives. That makes the Velociraptor incompatible with virtually all external hard drive enclosures, DIY NAS boxes, etc.