I assume (based on his first few tweets) that he'll be using it to promote his foundation primarily. I selfishly wish that he would use it less for self promotion and more for just odd ideas/thoughts, I think it would be really interesting to get into his mind occasionally.
actually he just tweeted about a blog he started documenting what he's learned through his foundation work:
If the original owner pretended to be Bill Gates (even as a joke, or as an imposter), they could legally take it away without much hassle. However, if it was just the URI stub for someone else tweeting as their real selves: good question!