If you have already written your django application, it may be really difficult to install it on Google App Engine, since you will have to adapt your data model. GAE uses big table, a (key,data) store, instead of a traditional relational model. It is great for performance but makes your programming more difficult (no built in many-to-many relationship handlers, for example).
Furthermore, most apps available for django will not work on GAE since these apps use the relational data model. The most obvious problem is that the great admin app of django will not work. Furthermore, GAE tends to make you use google accounts for identification. This can be circumvented but again, not using readily available django apps. This could be great for you, but it can be a hassle (for example, lots of user names are already taken at google).
So, my final advice is that, if you are a beginner, you should avoid GAE.
If you are based in Europe, djangohosting.ch is also a good choice, instead of webfaction.