Outlaw has said that they've got a center channel in development. I don't know what the time frame might be on that speaker, though.

It's hard to say what non-Outlaw speaker might match the Outlaw Bookshelf well since nobody's heard the Bookshelf yet. Of course, you could always get a third Bookshelf and use it as a center - after all, the ideal center is often considered to be one that is identical to the other fronts.