Heroku use it for their Doozer datastore which manages their distributed grid thingy.

Info here:

http://www.theregister.co.uk/2011/05/05/google_go/print.html

http://blog.golang.org/2011/04/go-at-heroku.html

http://xph.us/2011/04/13/introducing-doozer.html

And of course google use it internally.