If you want more information about it, read about it here (Parts 1, 2 and 3), but let's just say that it was a key feature that AMD had and Intel won't for about a year, maybe a bit more.
It was originally set for the Rev F Opterons coming out in about a month, but that looks to be delayed to G next year. This takes a major competitive advantage away from AMD for VMMs, moving them from unquestioned leadership to a far lesser lead. Word has it that in the VT vs Pacifica races, Pacifica will be ahead by a nose, but I'll wait for the benches before I proclaim a winner. µ
See Also
Intel spins Woodcrest launch