Microsoft does already have Live Mesh ( http://www.mesh.com/ ) which is a very similar product/service. Live Mesh does per folder synching and you can invite other users to share or read only on an individual folder basis.
They also acquired Foldershare years ago and it was remarkably similar, even had Windows/OS X integration, although they haven't done much with it since the acquisition. Dropbox feels like the much better designed younger sibling - the Web version is a first class citizen rather than some clunky admin panel, for example.