The link in the question had the answer... The location of `arcgisscripting` is defined in the Windows environment variable `PYTHONPATH`. By printing `sys.path` from a CGI script I could tell this was missing from the variables Apache was using. Easy fix: add this to `httpd.conf`:

`SetEnv PYTHONPATH "c:/path/to/pythonpath"`

And no more `ImportError`!