How do I get started with an Ubuntu friendly GUI framework?

Really the way to go overall for user friendly GUI app development is Quickly. It gives you a workflow and several helper scripts to get you up and running fast and easy. It is however very opinionated on the software to use, eg for GUI it uses PyGTK.

My own experience from looking into the Python GUI frameworks is that there is no single best one. Some say wxPython is the way to go, others PyQT. However I believe a lot of the Ubuntu specific software is done using PyGTK.

There is zenity for example.

As far as I know it does not provide a Python API but you can very easily configure the dialogs using its command line options. In fact it is actually meant for use in shell scripts but I don't see any reason to not use it in conjunction with Python.

For more information on all of its options I'd like to point you to the Zenity man page.

Here is a simple example on how to use it from a shell script.

This is only if you don't want to go with a more sophisticated solution. In general I'd recommend Quickly.