Just out of interest, is this costing megabucks for licensing the themes correctly? I'm under the impression that on a lot of sites (such as where you got your homepage theme ;)) you need to buy a special license if you're going to offer multiple users access to the same theme (e.g. akin to running a wordpress-mp site and letting users choose from themes you have purchased). I'd look into this now, as it's a fantastically well executed idea you're running with, and it'd be a shame for you to lose out as soon as you start gaining traction.