A Googler once told me that any code he writes during his employment with Google belongs to Google copyright-wise.
If they want to open source anything they have to ask Google for permission. It doesn't have to be hosted under the Google GitHub organization though (take Camlistore[1] as an example, although their primary repository is still hosted at googlesource.com).
This also means that if you want to contribute code to an open source project started by a Googler you will have to sign the Google CLA.