I'm looking into writing my first application of Google App Engine. C# is my 'native' language, and so writing the app in Java would of course be most logical. BUT, I'm a geek and would like to take to opportunity to learn something new.
Therefore its a toss-up between Python and Go.
The recently-released Go runtime for App Engine is labelled experimental for a reason: Both Go and Go-on-appengine are new and in a state of change.
If you want to experiment with Go and running Go apps in the cloud, go for it. If you want to write a production app on App Engine, use Python or Java.
I did a Python app for GAE recently and coming from C# I had no problems/was able to pick it up in a few days tinkering with the docs and playing with the SDK (I had some previous experience with python).
Python is pretty intuitive, it's imperative and OO, might require a slight change in thinking from C# but nothing revolutionary - using the interactive interpreter will allow you to pick it up in a day IMO. Learning how to use the datastore and figuring out other GAE specific API's (blobstore, image, mail) will probably be more work (it's specifics like transactions/groups, consistency modes ...) but you can figure that stuff out as you go/when you need it in your app.
One thing to note - recently announced pricing scheme makes Python a bad choice on GAE ATM because it cannot handle requests currently and this leads to one "instance" per request. This is also true for Go ATM (as far as I know). JVM OTOH can handle ~20 simultaneous requests per instance if I remember the mailing list conversations correctly. And you will be paying per instance/hour. This makes JVM the most practical choice if you plan on publishing your application in the near future.
Also using JVM you could use Scala (a new/cool language) and a framework like Lift which should theoretically allow you to port your code/avoid lock-in (disclaimer : I say theoretically because I'm only starting out with Scala and have no practical experience with Lift).
So I would recommend to look around the mailing list and see what other people are saying about the recent changes.