You may right that google hasn't released the source code of the latest android versions,but I feel it won't deny the developers to work with the latest versions.You will be able to get the latest rootfs images from the Android SDK , even it contains 3.1 rootfs images too.
Only Kernel's source has been released so far. 3.1 Kernel (2.6.36) is available at :
http://android.git.kernel.org/?p=kernel/tegra.git;a=summary
Since userland code is Apache licensed, Google can take its own time to release that part of code.
Does SDK emulate hardware similar to this tablet? If it does not that SDK image might not work.
-Shamit