Thanks for all the replies. They were useful. I think that my situation was best summed up by Mike - I need to figure out which things I have to do as root and which I have to do as me. I guess this only comes from experience, but it seems a good rule to follow.
My reasons for wanting the mac address are nothing to do with security. I have in mind a retail point-of-sale situation, where I want to record which transactions took place at which point-of-sale. I hope that, in this context, my use of the mac address will be safe enough. As for wxPython, it is correctly installed in /usr/local/lib/python2.4/site-packages. However, as part of the build process, wxPython builds its own version of gtk. It does not install it, but stores the generated files in the original build directory, and then hard-codes the path to the shared object files. As the original build directory happened to be under another user's home directory, my user could not read the files. I have deleted wxPython and reinstalled it from another directory which all users can read, and it now works fine. It was this 'other' directory that I was referring to when I asked if there was a 'correct' place, but I guess that anywhere reasonable is fine. Many thanks to all Frank -- http://mail.python.org/mailman/listinfo/python-list
