At first I started wanting to use GAE and a functional language. I was learning Lisp but I did not know about Clojure so my first idea was to use Python, but after some research and asking here on HN I found that Python is not really suited for FP. That's when I discovered Clojure (someone here on HN suggested it), since it compiles to the JVM, it works on GAE too.
Then I made some research for a while. At first I found only some blog posts on how to interact directly with the GAE API. I then came across The Deadline (https://the-deadline.appspot.com) here on HN, which is written in Clojure on GAE and it's a production app (I believe they do not use the library but a custom solution, but I really don't know). So I had the validation that it was possible and started coding the other parts of the project (I started with the iPhone app to have a proof of concept).
As you can see HN is a really good source for me. ;)
Regarding the editor I use Vim with colored syntax, autocompletion and rainbow parentheses. At the time I made some research and in the end I decided for Vim. To build the app I use Leiningen (appengine-magic has some Leiningen extension for building and deploying).
It works, but to tell the truth I really feel the need for an IDE for Clojure (and not some generic IDE like Eclipse or Netbeans adapted to Clojure using some plugins). I even thought about writing one myself, but for now I don't have the time and probably not even the knowledge.