In a statement released a couple of days ago on the English part of their Japanese website, Konami rubbished the idea of Pro Evolution 6 being an XBOX 360 exclusive. The statement reads:
It has been falsely reported on some websites that KONAMI will release its Pro Evolution Soccer 6 exclusively for Microsoft's Xbox 360. These reports are not true. KONAMI is currently proceeding with the production of a multi-platform release of Pro Evolution Soccer 6 so that as many people as possible can enjoy the game.
So Microsoft got it wrong (somehow) and it will indeed be a multiplatform offering. Konami have been very clever indeed by not confirming it for the Wii, but if they want as many people as possible to enjoy it, that would make sense. No doubt someone will confirm something very soon...