Tcl/Tk get sneered at by people who have never used it, but it is an incredibly productive environment if you need a GUI to drive a C program, especially with the new style widgets (ttk). Check out www.tkdocs.com. The downside is that Tcl isn't to everyone's taste. It has improved a lot with version 8.5 (e.g. tcl::mathop is much nicer than expr). Tcl can be made to behave a lot like LISP - you can add new control structures to the language, eval code, etc etc. See wiki.tcl.tk for more details.

If you are set on using Python, you could do a lot worse than wxPython. It is much more like "traditional" GUI programming (e.g. if you have done Motif you will pick it up straight away). You can also get at wx from C++ if that's your bag.