I haven't participated in a startup weekend but from following what has come out of our local one "AirBnB for x" does seem the be the most popular starting point.

I would agree with the author that the idea is unlikely to hit on something that will actually work, given the way it goes against economies of scale for what essentially be a commodity service.

At the same time though, I think it is good that people are exploring these ideas. Sure after a month or 2 (or even just the weekend) they might realise that the idea isn't going to fly. In doing that though they may have learned something and stumbled on an even better related opportunity.