Don't know when it will be released, but I wish they'd bloody hurry up. It's been held back for some reason. Perhaps they will release it after the summer, so they can add a couple of tracks from the tour??
You can get all the Greatest Flix really cheap now, especially on ebay. Check out this GFII if you're looking for another one until GVH3: link
There's also loads of others with great prices, if you just do a search: link
Almost anything on VHS is really cheap now!