AFAIK, there's a GWT 2.1 version "in the works" --you can get "Milestone" versions if you want to experiment-- and I think there will be a GWT 3 version too (sorry, I don't remember where I read about it). Google Wave died because of other factors -- but it shows a good sized application in GWT. Finally, being an OSS project, I guess it can still go on living, even if Google stopped work on it, but I hope it doesn't get to that.
Hope this helps!
Great fan of Open Source, Linux, and web development with GWT; all of these come together for my ESSENTIAL GWT book!
Google Wave isn't the only Google application utilizing the GWT framework. In one of the past Google I/O conferences (I can't recall the specific one), the speaker gave examples of how GWT is used for the Google ad words management tool. This shows GWT is embedded into a core business application so will likely continue to see some level of support.