Is there a way to uncheck all radio buttons in a group with PyGTK? No radio buttons are checked on startup, so I think there must be a way to return them all to that unchecked state.
There shouldn't be. By their nature, a radio button group is a 'pick one of many' type of control. From a human perspective, the idea is that one of them is always selected. The framework really should enforce that, so there really shouldn't be a 'none selected' state for the group. The none-selected state of a radio button group (in frameworks where it's possible) is very confusing to users, because that's not a state that they can get the control into.
If you want a 'none selected' state, I'd say you should add a 'none' element to the group, OR chose a different control type that conforms to what you want to do with it.