For those who don't know, Contributor's "thing" is that you pay anywhere from $1 to $15 per month to google, and they basically "bid" in ad spots for you. If you win the bid, then either the ad is removed completely, or a custom image/pattern/element is shown there instead.
Plus it gives you a rundown of where your money went each month, with site-by-site control (you can say only use contributor on these sites, or you can say use contributor on all sites except these). And at the end of the month if you haven't used your entire "contribution" of money, you get it "refunded".
Originally your contribution wasn't refunded, so if you "pledged" $5 a month and only visited a few sites, google kept the rest. They have fixed that, but i don't think they've made any "relaunch" posts or anything. Maybe they are planning something else before that?
Also it doesn't solve the issue of tracking scripts, so many people won't even consider it, and they are very vocal about it.
Oh, and it only works for google ads. So sites that don't use doubleclick or adsense aren't included.
Average CPM on a banner ad is $2.80. Let's say you can replace the four most obtrusive banners on pages by out bidding advertisers at a $3.00 CPM. Each pageview then costs you 1.2 pennies.
Of course, CPM's are going to vary wildly depending on the site, but the ad networks can determine this stuff easily enough and you could set your own CPM threshold.
A bidding war could be a scary scenario, but ultimately advertisers will be battling ROI as well and likely lose in the face of new competition.