Google is preparing for the launch of Google Music -- not that that's breaking news. What hasn't been talked about yet is the references to it in the marketplace code. There is also references to "OCEAN_BOOKS" (probably just books) and "YOUTUBE_VIDEO" (movie/tv rentals?).
I also noticed the URL of the marketplace redirects to http://market.android.com/apps/ when you go to the main page -- and going to http://market.android.com/music/ or /books/ doesn't give you an error like /charliesheen/ does.