When I switched to using mingw to compile the library containing my
extension functions, everything works fine.
This wasn't a trivial undertaking, and I'd certainly prefer to be able
to use MSVC to build the libraries, but I now have a working extension
library and that's the most important thing at the moment.
For the sake of anyone following later: If you're using postgresql 8.0
under windows and you want to build extension libraries, it looks like
you need to use mingw to build the libraries. You'll also need to
build a local copy of postgresql itself in order to get the
appropriate libraries and header files; it seems that the development
files distributed with the win32 installer aren't sufficient to the
task.
Thanks to everyone for the help,
-greg