>>> import sys
>>> sys.version
'2.4.1 (#65, Mar 30 2005, 09:13:57) [MSC v.1310 32 bit (Intel)]'
>>> import Tix
>>> import Tkinter
>>> root = Tix.Tk()
>>> root.tk.eval('package require Tix')
'8.1.8.4'
>>> root.tk.eval('package require Tcl')
'8.4'
>>> pw = Tix.PanedWindow(root)
>>> pw.pack(fill='both')
>>> pane1 = pw.add('pane1')
>>> pane2 = pw.add('pane2')
>>> pane1
<Tix.TixSubWidget instance at 0x008C79B8>
>>> pane2
<Tix.TixSubWidget instance at 0x009A00A8>
>>> pw.panes()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "C:\Python24\Lib\lib-tk\Tix.py", line 1244, in panes
    ret.append(self.subwidget(x))
  File "C:\Python24\Lib\lib-tk\Tix.py", line 337, in subwidget
    raise TclError, "Subwidget " + name + " not child of " + self._name
_tkinter.TclError: Subwidget p not child of 9953192

Documentation on Tix.* is sorely lacking, referring instead to the Tk documentation, which says: 
> pathName panes
>     Returns a list of the names of all panes.

The culprit appears to be the following code in Tix.py:
> def panes(self):
>     names = self.tk.call(self._w, 'panes')
>     ret = []
>     for x in names:
>         ret.append(self.subwidget(x))
>     return ret

This does not properly split the names list:
>>> pane3 = pw.add('pane2 pane3')
>>> pw.tk.call(pw._w,'panes')
'pane1 pane2 {pane2 pane3}'

The fix appears to be replacing "names = self.tk.call(self._w, 'panes')" with "names = self.tk.split(self.tk.call(self._w, 'panes'))"