The new site is now up and running (outside the SE network) here
PhysicsOverflow is meant to be some kind of a rebirth of the untimely passed away Theoretical Physics SE and a little physics brother of MathOverflow.
Compared to the former theoretical physics site, we have slightly lowered the bar to ask questions to graduate-level upward and broadened the scope to include experimental physics and phenomenology.
Apart from the high-level Q&A, PhysicsOverflow will offer in the future a Reviews section, dedicated to discuss and peer review (mostly ArXiv but other sources can be considered too) papers publicly and "in real time".
We would be very happy to welcome some of the nice people here, who are interested for example in theoretical/mathematical physics too, on PhysicsOverflow.