Main advertisement? I just went to android.com and developer.android.com; android.com advertises "Google built in" and lots of platforms, with a very small link to AOSP at the bottom of the page; developer.android.com has an AOSP link buried in its menus.
the 1st block is about the nexus one (market as open, but not on this page) and look! the second item on the page reads "Access to the entire platform source and information on how to contribute."
guess they forgot an asterix there saying that the "entire platform" means some of the platform.
