The idea would be to have a file checked by vdelivermail. If spam assassin is enabled, vdelivermail calls spamc before dropping the email into the users Maildir. Then the setup would be backwardly compatible with current sites. And they could enable/disable spam assassin by just updating the configuration file.
What I'd like to see with this is the ability to have either domain wide spam assassin settings or user specific spam assassin settings for the domain. I'm planning to host multiple virtual domains with Spam Assassin (mySQL Backend for configs) as an option, and having spamc called with the user name equal to the domain name would be great. I know some people would like a lower level approach so having vdelivermail have the option of passing no user name, the domain name as the user name, and the full email address as the user name would be helpful.