An interesting way to offer advertising, would be to offer to let people sponsor pages. This is done with Highways, (i.e. This section of highway sponsored by such and such). I think many users would like this idea, and would sponsor pages they contributed heavily to, and you could offer packages for "advertisers" outside of the project like mentioned above, (using some sort of algorithm to figure out what pages are relevant) and then offer a price per page type thing.
http://meta.wikipedia.org/wiki/Advertising_on_Wikipedia
-- Michael Becker