I am having the same problem across multiple domains, hosted at Fat Cow.
I use one domain as a test site only, so today, I reinstalled WP3.5.1. and deactived all plugins. It defaulted back to Theme 2012, but I still cannot preview it, or any other themes. The preview box is simply blank. Since it is happening across multiple domains, I contacted my hosting company, they poked around, and said it must be an issue with WP, not a server problem. This is very frustrating, as I am looking to change themes on one of my live sites, but without preview and the ability to change the theme options I cannot move forward.