Google is still very committed to providing a great place to host open source projects at http://code.google.com/hosting, that has not changed. But many of us are also fans of GitHub, and recognize that there is a great community of developers there (particularly in the ruby community).
The Google account on GitHub has been setup simply as a place where some open source Google projects can be mirrored. As another commenter noted, this is actually one of the points of a DVCS.
What's the model for contributing patches going to look like? With GitHub projects, the usual method is to create a fork, do some work, and send in a pull request with a descriptive note. If it's easy to contribute, more people will do it; this is one of the big reasons why so many people are fans of GitHub.
Will contributing to the projects mirrored here be that easy?
"But many of us are also fans of GitHub, and recognize that there is a great community of developers there (particularly in the ruby community)", that's an interesting comment. Does Google care about Ruby?
While it doesn't get as much use as other languages internally, there certainly are a number of in-house projects that use it.
But more importantly, developers accessing our public APIs use Ruby. We maintain a number of client libraries to make accessing our APIs easier. (We also maintain similar libraries in Objective-C and C#, for example.)
The Google account on GitHub has been setup simply as a place where some open source Google projects can be mirrored. As another commenter noted, this is actually one of the points of a DVCS.