The current undo is a simple 1 level undo for the "code" tab.
Right now I'm deep inside the USF coding and I cannot really think about it until before the release of 4.0, but i will add it to the todo "list to consider". The major paint is how deep it goes which means that it will require a massive rework on the current engine, and the memory requirements will jump up considerably too. and probably changing anything in "code" tab would have to reset the "undo" history, as i'm not really aware of what user changes there (e.g. copy/paste).
btw, Photoshop 4 did not have more then 1 undo level too, still beeing succesfull