It's a neat mix and match of new source code, binary files from Gingerbread, bits from the Galaxy Nexus, and custom code from other developers. Android dev kwiboo has done an excellent job getting things going, and even facial unlock is working -- as long as you don't have a big bushy beard, that is. As someone who has also been fooling around with the code and flashing my own Nexus S at least a jillion times, my hat's off to him -- I'll say it again, he's done an excellent job.
Source: XDA-Developers.