This month's copy of Nasa Tech Briefs (dead tree edition) mentioned the Tweel, Michelin's new tubeless tire. It has a rim and flexible spokes. The CBS article above mentions that it gets a bit noisy at high speeds, and it isn't in production yet, but it certainly looks promising--they say it handles well. Number two daughter thought of one issue immediately, though--what happens if you get a stick through the spokes? I guessed that you could cover the empty space with some flexible plates (maybe rubber) to keep that from happening accidentally, but that's a good question.