There is an SDN member who is working as a hospitalist so we can see if she sees this and posts.
I was talking with someone here the other day who is planning on doing a 2 year fellowship as a hospitalist at our place. They said that there is no certifying board yet, but that it was expected in the future. It'll be interesting to see if this happens and if so, if it becomes a 3 yr research requiring field. I'd guess not.
Like many such areas, I expect that in the future some type of fellowship will be the norm or at least would open up the most job opportunities. I would think that there would be a long period of time when lots of good jobs for those wishing to do this would be available without the fellowship though.