I think it's very possible that the starting center is not yet on the roster. Drafting someone who would start at the 5 seems unlikely, but free agency and trades seem very likely.
Also, call me crazy, but I wouldn't be surprised if Oberto is released, resigned for somewhere around the minimum, and ends up starting some games throughout the year.
I also think every single one of the Spurs big men (minus Duncan) could be traded. Barring trades, however, I think you're on the right track with Bonner/Ian/Gist being the order of the big man rotation.