There is no reason why you shouldn't use GAE and Google Web Toolkit (GWT) together. You write your backend code in Python and the frontend code in Java (and possibly some JavaScript), which is then compiled to JavaScript. When using another AJAX framework you will also have this difference between server and client side language.
GWT has features that make remote invocation of java code on the server easier, but these are entirely optional. You can just use JSON or XML interfaces, just like with other AJAX frameworks.
GWT 1.5 also comes with JavaScript Overlay Types, that basically allow you to treat a piece of JSON data like a Java object when developing the client side code. You can read more about this here.
Update:
Now that Google has added Java support for Google App Engine, you can develop both backend and frontend code in Java on a full Google stack - if you like. There is a nice Eclipse plugin from Google that makes it very easy to develop and deploy applications that use GAE, GWT or both.
