Western Digital has recently expanded their VelociRaptor line, adding more customization options tailored towards servers or high-end workstations. They are adding a new version of the 300GB, 10,000RPM drive that is compatible with backplanes, common in servers and sometimes seen in workstation for hot-plug RAID arrays. In this instance, Western Digital is adding a 3.5" mounting frame with a built-in heat sink that can be placed into traditional 3.5" bay. They are pitching it to enthusiast machines too, not just servers using backplanes.
The latest VelociRaptors are certainly fast drives, spinning at 10,000RPM, having ample cache and squeezing a fair amount of space into a tiny 2.5" form factor. Definitely not a choice for laptops, though that is traditionally where 2.5" drives go, but they also have a place in server environments where power consumption is an issue and space comes at a premium. You can read more in the full press release on the WD site.