You'll probably be able to choose. Hopefully. Or maybe it's just completely random. (Blue grass FTW.)
Watching the TED video it certainly looked that way to me (choosing, that is). It looked like you can pick a planet then pick which stage to start with. A bit like choosing a neighborhood and lot in Sims (and family).
Edit: You know what, I take that back. I have no idea. If you were able to pick and choose your planet, that would mean all the planets would have to be generated and set at the get-go, or else how could you view the planets in order to pick one? Maybe you pick a system or galaxy like the lots in Sims, one of your families per lot, for example. Then it puts you on a planet somewhere in there. The planet would be generated at that point, randomly but within certain limits.