There is an experimental release of Go for Google's App Engine: http://code.google.com/appengine/docs/go/overview.html. It's mature enough that it was used in production to generate the Thanksgiving 2011 Google Doodle: http://blog.golang.org/2011/12/from-zero-to-go-launching-on-....

They Go App Engine team have also very recently pushed out beta releases that are tracking the weekly Go releases leading up the final Go 1 release, so I think it's safe to say that Go will be officially supported on App Engine not too long after Go 1 is released.