KT400a's was just announced yesterday, and boards with this chipset should be looming around the corner.
A tweaked memory controller is one of the few things they added to this revision. As of right now, boards with the Nforce2 chipset perform much better than KT400 boards, and the KT400a looks to fix that.
Soyo makes no mention of it, but theres
this link with a picture of the Soyo KT400a near the bottom, so I think its safe to assume a Soyo KT400a board is emminant.