The Inq-well's got the details on the half-step update to the SATA specification.

"The specs, SATA 1.0a, and the six post SATA 2.0 extensions, are now collectively called SATA 2.5. SATA 1.0a is an minor addition to SATA 1.0 spec, and as with most things with an 'a' extension, it cleans up problems at 1.0. The post 2.0 specs are a little more interesting."