Thanks for your help, Robin. I got wxPython 2.2.7 and it builds
and runs like a charm with wxGTK 2.2.7.
There is a line in the BUILD.unix.txt file about this, which I glossed
over the first time:
B. You'll usually want to use a version of wxGTK that has the same
version number as the wxPython sources you are using. (Another
advantage of using CVS is that you'll get both at the same time.)
I didn't make the connection because it was in the wxGTK section, not
the wxPython section. The word "usually" is perhaps not strong enough
for dense people like me 
By the way, is wxPython 2.3.0 compatible with wxGTK 2.3.1?