I'm working on a plugin which will automatically affiliate any links that users post when there is an affiliate link defined in your database.
I've just started beta testing it on a small number of stores on FilmJournal.net - I'll be adding more as I develop the plugin and once it's ready I'll release it for everyone to use.
I haven't done anything on the admin-side of the plugin yet - that'll come once I'm happy with the way the plugin works on the blog side :)
More details on it are here and as soon as it's ready I'll make it available to download.