Here’s Geoff Johns, interviewed for Newsarama, May 2007, about the 52 parallel worlds just unveiled at the end of the story of 52.
"Right now, they're just out there. 52 earths. That's all. And you'll start to see them here or there, but the goal really is, like the end of the issue said, 'It's a world full of possibilities.' We don't want any rules on our stories. So let's be able to tell stories of all sorts of different things."
DC Comics has decided to list them instead, also on Newsarama.
DC editorial: where they believe that butterflies are much prettier after they’ve been gassed and pinned to a board.
Doom Patrol “The House That Jack Built” by Grant “And no crossovers! Each of the parallel universes should exist in its own separate stream with no contact from the others…” Morrison (writer), Richard Case (penciller), Scott Hanna (inker) and John Workman (letterer), Doom Patrol Issue 24, July 1989, reprinted in Doom Patrol: Crawling from the Wreckage, DC Comics, 2004