With a statement such as
ErrorDocument 404 /wiki/cgi-bin/ikiwiki?do=create
in apache's configuration, I think that it would be possible to let the user surf to non-existent pages and be prompted to create an entry, as it is with other popular wiki engines.
From the apache documentation, it seems that the environment variable REDIRECT_URL will carry the name of the page the user has accessed. Also see ErrorDocument's documentation.
Nice idea, I'll try to find time to add a plugin doing this. --Joey