The only fully portable GUI for Python is the standard TkInter , unless you need an extra installation next to Python. Themed Tk version is pretty nice compared to the old version of Tk (the thematic version is available through the ttk module).
A few weeks ago I had to answer the same question as you. I came to the conclusion that PyQt is currently the best choice for a modern, powerful, well-groomed and portable GUI, mainly due to some shortcomings of its main rival (wxPython, see below), (Tk and Themed Tk would be nice for simpler needs.)
Two words of warning from wxPython : it cannot be installed through the popular Fink package manager on Mac OS X, which currently makes it less portable than PyQt and TkInter; As far as I know, it is not yet compatible with Python 3.
PS (December 2012): PySide is currently a strong alternative to PyQt. There are several Stackoverflow questions about the respective merits of these two Python bindings.
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