Perhaps the best solution would be like the one Salon, Weather Underground, and many other sites use: pay a subscription fee (say, $29.95 a year) or see (and probably ignore) the ads. That way we'd have our choice of how to personally support Wikipedia.
jpgordon
I agree with the idea of the subscription fee if you can't stand even discreet Google Adsense type ads. But instead of $30 a year, why not $5?
All the reasonable advertising proposals I've seen have suggested $0/year. IOW, if you want to turn off the ads, you click a button which says "turn off the ads". If you're logged in, this turns off whenever you're logged in. Either way, a cookie is set to turn off the ads as long as you still have the cookie set.
And any relatively active user is probably going to generate $5 worth of Adsense ads anyway, isn't he?
The standard adsense program only generates revenue when an ad is clicked, so there would be lots of active users that would generate $0.
I don't know what percentage of users donate currently, but it's probably very low. Even assuming that all these people drastically reduce their contributions in reaction to the introduction of ads, you'd probably be a long way ahead.
Steve
Almost all users donate/contribute, they just don't do so monetarily.
I think the only proposal right now which has any chance of reaching something nearing a consensus is to put ads on a separate site, such as nupedia.org. The WMF could create a wholly owned for-profit subsidiary corporation, give it the nupedia domain name, and give it a live feed to the database. This plan wouldn't make nearly as much money as putting the ads on wikipedia.org though, at least not at first.
Anthony