Expect April if you want Thoroughbred in a notebook, late May, June, or possibly July (10% May, 60% June, 30% July) if you want it for a desktop. The next Athlon processor, as indicated on AMD's latest roadmap, is the Palomino 2200+ coming in late March or early April. The Thoroughbred should appear two to three months after that. Both AMD and Intel generally follow three month cycles, unless forced to do otherwise.
I expect the KT400, also known as the KT333A, to be released around the same time.