Actually iOS 6 does introduce a set of new APIs for attributed strings that would make something like this a lot more performant and obviate the need to use web views. I wouldn't be surprised if that was one of the driving factors in making that the minimum target.
Well, a lot _easier_, anyway; you could in principle do it with Core Text before, and Marco probably has experience in this. He did mention on his podcast that he'd consider an iOS5 version if iOS6 exclusivity is a problem.
