While it is possible to compile 64 bit python on Mac OS X, the default
tcl/tk is NOT 64 bit.  If you do install a 64 bit version of tcl/tk,
python still will not find it - lines 1503-1515 prevent it:

        if 'x86_64' in archs or 'ppc64' in archs:
            try:
                archs.remove('x86_64')
            except ValueError:
                pass
            try:
                archs.remove('ppc64')
            except ValueError:
                pass

            for a in archs:
                frameworks.append('-arch')
                frameworks.append(a)


I was able to successfully get 64 bit tkinter working by commenting out
these lines as suggested by Ned Deily.

Ronald Oussoren suggests:
"BTW. The correct fix would be to add a probe to setup.py that detects
if Tcl/Tk was build with 64-bit support or not."

Adam: where can I download a 64-bit version of Tcl/Tk?

Sorry Ronald, I had misinterpreted your last message.

Look at these posts:
http://buffalothedestroyer.blogspot.com/2009/07/installing-64-bit-tcltk-on-mac-os-x.html
http://www.nabble.com/Error-compiling-tk-8.5.7-on-Mac-OS-X-10.5-td23790967.html

or try this script:
curl -O
'http://osdn.dl.sourceforge.net/sourceforge/tcl/t{cl,k}8.5.7-src.tar.gz'
for f in t*8.5.7*.gz; do tar zxf $f; done
cd tcl8.5.7/unix/
./configure --enable-framework --enable-64-bit
cd tk8.5.7/unix/
./configure --enable-framework --enable-64-bit
make -j 4 -C tcl8.5.7/unix
make -j 4 -C tk8.5.7/unix
sudo make install -C tcl8.5.7/unix
sudo make install -C tk8.5.7/unix

I found an easier way to test with a 64-bit Tcl/Tk: run Snow Leopard ;-)

The (untested) attached patch ("setup-tk-archs.patch") updates the code in 
setup.py to automaticly detect the architectures that are supported by the 
Tk framework and passes '-arch' flags to the compiler for the intersection 
of the configured architectures for Python and the architectures supported 
by Tk.framework.

I just commited a fix for this, the patch is present in all 4 active 
branches (2.6, 2.7, 3.1, 3.2)