You can find the kernel at http://git.android.com
and the other
mirrored GPL and LGPL'd components at http://code.google.com/p/android/downloads/list
.
Notices for other licenses can be found within the SDK.
Over time, more of the code that makes up Android will be released, but at this point, we have been concentrating on shipping an SDK that helps application developers get started. In short: Stay tuned.
One of the best explanations for the reasoning behind releasing code under Apache2 can be found in a ArsTechnica article by Ryan Paul.