Most of it uses the Apache Source License 2.0, which doesn't require releasing the source. You can look through http://android.git.kernel.org/ (especially external) for some others, but they're mostly similar licenses.

There are some GPL things in there besides the kernel, and a few of them are probably shipped. I'm not sure how e.g., Motorola is handling this (since they clearly have an obligation to provide the GPL source).

See also http://source.android.com/source/licenses.html

My understanding is that all the exciting changes in Honeycomb involve the user interface, which is all ASL.

WebKit is (mostly LGPL v 2.1