By the way, if Morrison's GL doesn't sell or is badly received -- neither of which seems likely based on first issue feedback -- DC would simply take Morrison off the book, replace him with a different writer, and save face by saying that Morrison's schedule doesn't allow him to continue. The new writer would quickly wrap up the story and they'd move forward. There wouldn't be an out-and-out cancellation under any circumstances.
It's kind of what they did years ago with JMS when he took over both Superman and Wonder Woman and both were poorly received.
But again, based on Morrison's existing fan base and the positive reaction to #1, I'd predict his 12 GL issues to all land solidly in the Top 50 each month if not the Top 30.