Scenario to reproduce the problem:
- Run the attached script.
- Click the 'Ask confirm' button and answer 'Yes'; it should print 
True, which is the expected answer.
- Click the 'Ask file' button, select any file and confirm.
- Click the 'Ask confirm' button and answer 'Yes'.
The script prints False, which is obviously wrong.
Problem reproduced on Linux Red Hat Fedora Core 4, Suse Enterprise 
Linux 9, Solaris 8 for Sparc and Solaris 10 on Intel. The script works 
as expected on Windows 2000, so it seems to be Unix-specific.

Possible cause: the result of the _show function in tkMessageBox is not 
always a string, apparently depending on what happened before. Changing 
the last line to:
return str(res)
seemed to correct the problem for me.

I've verified it, it is indeed a bug in tkMessageBox.py which is very
likely to be noticed while using tk 8.5 or newer.

I guess this will have to be accepted without any tests, unless someone
can come up with a way to test tk_messageBox under Windows and Mac.

I've also confirmed this one (by chance).  I'm on ubuntu linux, and I am seeing the problem in both 2.6 and 3.1, both using Tk 8.5.  I don't see the problem on my windows install (Vista, python 2.5, not sure of the Tk version right now).  But I did find a hack around this by explicitly creating and destroying a top level window following the askdirectory() dialog.  Try this snippet on linux/unix, the call to askyesno() returns False always.

import tkFileDialog,tkMessageBox,Tkinter
theDirectory=tkFileDialog.askdirectory()
addDirectory=tkMessageBox.askyesno('a dialog','Add a directory?')
print addDirectory


But the following works fine:

import tkFileDialog,tkMessageBox,Tkinter
toplevel=Tkinter.Tk()
theDirectory=tkFileDialog.askdirectory()
toplevel.destroy()
addDirectory=tkMessageBox.askyesno('a dialog','Add a directory?')
print addDirectory

the patch works for me with tcl8.5

fixed for 2.7, 3.1, 3.2, commit pending for 2.6 until after the 2.6.5 release

committed to the 2.6 branch as well