I've been glancing at stackoverflow.com (listening to their podcast as well) and participating in both onstartups.com and startups.com - having worked with non-profits before, here are some potential revenue possibilities as a community of like minded folks get together.
While I probably wouldn't take medical advice from a site in this format yet, as we find members on these site that contribute their time and references, this is the best system I've yet seen to harness community knowledge.
Well I can tell you exactly what the deal is for THIS site!
This is like blogging. It's almost impossible to make money off ads and blogging, even with thousands of readers (you need many 10s of thousands). But blogs can be a fantastic way to drive traffic to other things that do make money, like a startup or like consulting time.
Let's do some estimations:
For StackOverflow (a growing website) with more than 100K worldwide UV by day and 4M UV/month (see adPlanner stats ), one can pick a reasonable average CPM of 0.01$/UV/month then the monthly estimated revenue is equal to : 40 K$ /month. That is a pretty amount of money.
Thereafter, you can add every additional source of revenue given by Alex plus some other revenues specified in this thread.
And finally, you can conclude that StackOverflow is a running business (even if we don't know their operational costs...we can estimate them as well knowing that they are a company of 3 people).
Let's say that I totally disagree with Oleg's answer :-)
Concerning answers.onstartup.com, it is more difficult to guess since traffic is lower and monetization ways are not so manifold. But I don't think that it is a business losing money.
Answer to #1: I'm not an owner, but I don't think they do have any revenue yet.
Answer to #2: I think it's close to impossible. Your costs: monthly fee to Stack Overflow, hosting, some moderator(s) who keep the site clean and lean. Your revenue: ads? Forget about it. There won't be nearly as many pageviews required for ad revenue. Nothing stops everybody and his dog from opening an exact replica of your site, thus driving pageviews and potential revenue off your site.
I think that even Stack Overflow itself won't be a hot darling for a long time. It's relatively easy to develop. There will be competitors soon. Then there will be a sourceForge variant (i.e. free) too. End of opportunity.
Please take a look at Stack Overflow. Revenue is obtained through advertisements, sponsoring of badges and other means. For instance, a start up magazine might eventually be interested in sponsoring a badge here .. meaning the badge would become the "magazine name" badge instead of the "Taxonomist" badge.