My son has given his verdict “Scratch is great, Dad, and you can do all sorts of stuff with it, not just games. Kodu’s just a way of making games. It’s not as fun.”
Digging around a bit, it seems that MS is making the source code available for academic institutions to look at, but it’s not being released under an open source licence:
“Shared source” arrangements can potentially be quite dangerous for the licensee, as Larry Rosen points out
Scratch source code is, in contrast, available under a licence which is getting on for being an open source licence, but isn’t quite (see here for details). The non-commercial restriction prevents it from qualifying as an open source (or free software) licence, but it’s certainly a fairly major step in the right direction.