I'm going to take a more unusual view about advertising here - Wikipedia is a reference site, people come here looking for information, and if we link over to relevant (and that's the important thing) adverts from each article, we're actually enhancing the service we offer for other users.
Going for targetted adverts - there's ways to "buffer" what appears alongside an article and the actual adverts by forming the adverts into, say, a popup window, with a link from the side of the article, or some form of Click to see Relevant Adverts type script and so on. Relevant Adverts are available to our readers, but they don't see them unless they want to see them. This, of course, is going to impact on the ability to earn money, but WMF isn't needing every last penny it can lay it's hands on, a modest income is best, it makes the Foundation look like it's interested in covering it's costs and not grabbing every last penny.
The other option is generic adverts, who's particularly bothered if they see an advert for a film, a CD, a mobile phone or a car when they're browsing some non related topic. There's not the same problems with conflict of interest when you're browsing an article on, say, Windows XP, and an advert for a BMW appears.
Just my thoughts though.