tkFileDialog.Open is broken, apparently due to the new object interface.

Try the following:
>>> import Tkinter
>>> import tkFileDialog
>>> root = Tkinter.Tk()
>>> tkFileDialog.askopenfilename()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/lib-tk/tkFileDialog.py", line 119, in askopenfilename
    return Open(**options).show()
  File "/Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/lib-tk/tkCommonDialog.py", line 54, in show
    s = self._fixresult(w, s)
  File "/Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/lib-tk/tkFileDialog.py", line 89, in _fixresult
    if not widget.tk.wantobjects() and "multiple" in self.options:
TypeError: wantobjects() takes exactly 1 argument (0 given)
>>> 

Looking at the code, tkFileDialog.Open _fixresult is calling widget.tk.wantobjects() and apparently that function always wants an argument (though one would not necessarily think so from the function name, so the bug is not surprising).

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What version of Python are you using?  I just looked at the
code for 2.3.2 and tested it.  2.3.2 works fine.  Perhaps
this bug has already been fixed?

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This has been fixed in _tkinter.c 1.161, so you are probably
using some old version.

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Sorry I forgot to say the version. I am using Python 2.3.0 (via the binary installer MacPython-OSX-2.3-1.dmg). I'm very glad to hear that this has already been fixed.