After a while, I became aware that subversion was also upgraded. Building pysvn again proved to be a difficult task. I upgraded from python 2.5.1 to python 2.5.2 in order to try to also install the development libraries required to build pysvn from source. I had many issues there and then decided to move on and just install subversion 1.6.1 and the python 2.6 egg from the pysvn website.
I then discovered that lxml wasn't installed properly, and every time I tried to install the egg it told me that lxml was a pre-requisite of lxml... a bit of a confusing error message, but basically the change came about because 10.6 builds for 64 bit by default. Eventually, after attempting to build lxml for quite a while, I discovered the issue was that it wasn't working with gcc-4.2, the default compiler. If you get an error like this:
/Developer/SDKs/MacOSX10.4u.sdk/usr/include/stdarg.h:4:25: error: stdarg.h: No such file or directory
lipo: can't figure out the architecture type of: /var/folders/....
Then you have to do two things.
First of all, make sure you install the new XCode including the XCode 10.4 SDK (an optional install that is unchecked by default). For some reason most of the python modules I've found seem to rely on the 10.4 SDK and they even back-port the install for 10.3.
Secondly, you have to change your default compiler from gcc 4.2 to gcc 4.0. This can be done by removing the symlink file in /usr/bin/gcc and re-linking that to gcc-4.0. This can be done by doing:
sudo rm /usr/bin/gcc
sudo ln -s /usr/bin/gcc-4.0 /usr/bin/gcc
After all that is done, you should be able to download lxml and build it using the standard Mac build command:
python setup.py install --static-deps