The Sigma 50-150mm f/2.8 II, announced in December 2007, is an update to its previous 50-150mm f/2.8. It's hard to tell exactly what has been updated: comparing the specifications of the two lenses, they appear almost exactly identical, except version II seems to have shaved off 8mm from the overall length.
The difference appears to be in the mount options, as Sigma has made HSM-style versions available for the previously unsupported Pentax and Sony lens mounts. This is good and bad news: while the lens will mount on those bodies, if the body only supports mechanical (screw-driven) style focusing, autofocus will be unavailable. Newer Pentax and Sony bodies will support the HSM standard. Read the whole review here. You can buy the lens here