We should think about the advertising model before its too late and it gets shoved onto us. I think each community should get its own advertiser and they get a stick post on each post which is limited to 300 chars and one url.
Its simple, non intrusive and we can make it a bidding. 80 % of the proceeds should go to the maintenance of the instance, 20 % should go to the moderators of the instance.
Would warner brothers want this for 100 $ on movies ? I think so ..