See coverage of knol here and here. It’s incredible they can’t even bring themselves to mention wikipedia in their own announcement. To be fair wikipedia has stagnated in that the mediawiki software hasn’t changed much in years, and the anti-advertising stance is pretty short sighted and just leads to big ‘save wikipedia, donate!’ banners every 3 weeks. But. Still. Why kill the puppy?
Update: Ok so it’s more competition, but Google engages with Firefox to the point of basically running it. So, why not contribute to either the wikimedia foundation or add features to mediawiki? And ok if they tried that and it didn’t work out… but there’s been no meniton of that.