Not sure if this is an issue for others but I have a hard time reading openstreetmap tiles on retina displays. Decided to take a stab at making my own. I recently found the cartocss file for openstreetmap tiles on github, loaded it up on tilemill, and linked it to PostGIS. Then I set the scale factor to 2, and rendered retina tiles!
I tweaked Bryan McBride’s (@brymcbride) php code for serving the MBTiles from my server.