A great site for people learning scripting, contains a whole host of stuff:
- maya api and mel sections
the help section in maya is also extremely useful for learning how to build a gui. when you boot the help up scroll all the way down to 'commands' or 'commandsPython' in the left frame and start learning.
if you dont mind what language to use (mel or python) then my bFlocking script in my signature uses python and builds a window with various ui controls such as frames, buttons and input boxes etc.