There are two symptoms here, each caused by a different bug.
Neither is specific to IDLE; they have to do with Tkinter recognizing 
the correct Tcl/Tk installation at build time and run time.

Bug 1: The Python 2.6 binary installer appears (judging from the error 
message) to have been built against a Tcl installation in 
/Library/Frameworks/Tcl.framework. Mac OS X (10.4.11 and I assume 
Leopard as well) comes with Tcl/Tk, installed in 
/System/Library/Frameworks, but there will be no 
/Library/Frameworks/Tcl.framework unless you install Tcl/Tk on your own 
(but read on before doing so).

Symptom 1: Hence, the error about not being able to find 
/Library/Frameworks/Tcl.framework/Versions/8.5/Tcl. This symptom should 
not manifest if either a) you install Tcl/Tk 8.5 on your own, or b) you 
build Python from the source (so that it finds the pre-installed Tcl in 
/System/Library).

Bug 2: The Python source distribution (both 2.5.2 and 2.6; I have not 
checked earlier versions) searches for Tcl and Tk in the wrong order, as 
pointed out in the comp.lang.python post referenced in msg74440 and 
msg74544.

In Mac OS X's standard search order for frameworks, /Library/Frameworks 
comes before /System/Library/Frameworks (see the ld and dyld man pages: 
http://developer.apple.com/documentation/Darwin/Reference/ManPages/man1/
ld.1.html and 
http://developer.apple.com/documentation/Darwin/Reference/ManPages/man1/
dyld.1.html), whereas the setup.py script in the Python source 
distribution tries /System/Library/Frameworks/Tcl.framework before 
/Library/Frameworks/Tcl.framework (and same for Tk.framework).

Symptom 2: This causes a problem _only_ when you have a separately 
installed version of Tcl (e.g. 8.5).
Python's setup.py script finds the Tk installation in 
/System/Library/Frameworks/Tk.framework (version 8.4) and uses its 
headers, whereas the linker (being part of Mac OS X) chooses the Tk 
dynamic library in /Library/Frameworks/Tk.framework (version 8.5 if 
that's what you installed). (I suspect the linker records the Tcl and Tk 
installations (or versions) found at build time.) Hence the error 
message about Tk version mismatch.

If you are affected by both of these bugs, Tkinter will not work whether 
or not you install Tcl 8.5 in /Library/Frameworks.

I believe the setup.py script ought to be corrected (as per the post 
cited in msg74544). I do not know of an easy fix for existing Python 
installations that are affected by this, other than rebuilding Python.