In a post on the Android-building Google group, Dan Morrill announced that the AOSP tree (the "Master" and "Gingerbread" branches) are once again available online. Because of problems with the kernel.org servers, the source has been MIA for a while, and in the downtime Google ramped up work for their own git hosting on Google's servers.
For now, only the code is available. Google is working hard to get the Gerrit code review site back in operation, then gitweb afterwards. Dan also reiterates that the Ice Cream Sandwich source code will be available as soon as devices running it are delivered.
With new servers comes new download and building instructions, which Dan outlines in his message at Google groups. Be sure to have a look and get your new repository set-up -- you can find all the information at the source link.