Western Digital's VelociRaptor replaced its Raptor series of HDDs in 2008. It cut the form factor down to 2.5" and pushed the capacity up to 300GB and still gives you about the closest thing you can get to SSD performance from a traditional hard drive today (at least in terms of access times).
The updated VelociRaptor will be launching later this quarter and keeps the 10,00RPM motor and 2.5" form factor. You will also still have the option of the "IcePack" 3.5" mounting frame (which may again prove redundant unless you haven't got somewhere to mount 2.5" drives) and have capacity options of 150, 300, 450 and 600GB. The interface gets bumped to SATA 6.0Gbps, along with a claimed latency of 3ms and sustained transfer speeds of 145MB/s. There is currently no word on what you can expect to pay for the drives once they hit retail (VR-Zone is claiming that will be by June).