My thoughts.
The MacBookThin/Alu NEEDS Leopard so not until Leopard appears - so maybe soon. Would rock but would also need to have an Apple event to promote the release.
But I think more likely MWSF 2008. I figure they maybe will put a 32GB flashdrive in it to keep it thin so longer they leave it the cheaper because of the current constraints on flash. More time to iron out bugs as well if they also also go the new touchpad/touch?