I can't help pondering about the implications this move could consequently bring to the perceptions of net neutrality. Not that I see this project as something inherently bad, it is just the contrary, but it can give rise to some bad moral precedents.
Would you care to expand on these bad precedents that you are referring to?
What if google or bing made a similar deal? Or any large corporation, for that matter? You set up an internet where the majors can buy their way into a monopoly position, not by convincing ISPs to degrade other services, but by convincing them to charge users less to use your service. This makes it harder to impossible for new companies to enter those markets.
All that being said, I think this (Wikipedia Zero) will be a net-positive thing, and I hope they succeed. I do see that this type of thing could have worrying implications for net neutrality, but ultimately they're not the first site to broker "free access" deals with mobile carriers.