Just poking around, I find a list of IMAX theatres which will be showing The Hobbit in HFR (High Frame-Rate) 3D:
and another page for finding which theatres near you will be showing The Hobbit in IMAX 3D:
Not sure this represents anything like 500 screens' worth, though, but I imagine we'll be getting more updates, the closer we get.
Edit - from TOR.n:
Complete Listing of Theaters Showing Hobbit – HFR 3D, IMAX 3D, IMAX, Dolby Atmos
(includes link to a downloadable PDF version)
About HFR: http://www.firstshowing.net/2012/wb-...r-3d-with-faq/