UPDATE: The site has reached 100% commitment, and is about to launch a beta Q&A period.
There is an active proposal at StackExchange to build a research-level Q and A site for theoretical physics. There is a Physics StackExchange site already, for questions at the high school or undergraduate level. The new site would be expert-level only, like MathOverflow for math, or CSTheory for theoretical computer science.
The new site is currently in the “commitment” phase, meaning more people need to add their names to a list of individuals who want to participate in the site when it does live. If enough people commit (at 79% of requirement as I write this) the site will enter a “private beta” phase, where only those who committed will be able to ask and answer questions for a while.
More information, and the commitment page, is here.