Twitter is Apples preferred social network. the integration baked into Mountain Lion is not surprising to assume that iChat might eventually get built in support for Twitter DM's.
I mean why not? Twitter is already increasingly getting exposure on the social network. It'd be nothing technically to include a plugin for it. The Direct Messages would be the ideal feature to incorporate into iChat/Messages.
It would be one step closer to a complete communications package that I dream of and predict will eventually happen. Its ok for these corporations to wish us to reside completely in their walled garden ecosystems but communications such as real-time chat with audio or video capabilities screams to be open and cross-platform; its in its nature.