Are there any UI widgets available to the python side of Google App Engine? I'd like something like the collapsed/expanded views of Google Groups threads.  Are these type things limited to the GWT side?
There is no difference in the amount of built in widgets available to the python and java sides of app engine.  Neither side has any!  App Engine is primarily a back end technology. It allows you to use pretty much whatever web framework you want for your presentation layer, subject to constraints that Alex mentions.
GWT is completely unrelated to App Engine, besides being developed by Google.  It is a client side toolkit, and can be used just fine with any web app as a backend, whether created in java, python or [your favorite language].  (Admittedly, you get a few bonus features if your backend is in java.)
Google App Engine UI Widgets