Solid cross-platform threads support in GUIs: tkinter, pygtk, wxpython or pyqt

wxpython pyqt tkinter pygtk python-multithreading

For wxPython, check out the following links:

http://wiki.wxpython.org/LongRunningTasks

http://www.blog.pythonlibrary.org/2010/05/22/wxpython-and-threads/