This posting is just for those who hadn't yet seen it. Max Tegmark views our universe as one among many possible mathematical structures, and he believes that all mathematical stuctures are indeed manifested physically, though only a minority contain observers to testify to their existence. His Multiverse FAQ can be found here. Tegmark's conclusions have big anthropic implications, the likes of which we're only just beginning to unravel. He also wrote a paper on why anthropic considerations force all intelligent observers into universes with three space dimensions and one time dimension.