As far as I can work, out, TOWERS OF MIDNIGHT could come out sooner than Fall 2010, at least 3-4 months sooner, in fact, but this is only possible because Brandon had half of TOWERS OF MIDNIGHT finished when he had to go back and revise THE GATHERING STORM. So TOWERS could be out in the summer of 2010 but because he still has to write A MEMORY OF LIGHT from scratch (bar notes and RJ's completed scenes etc), we'd likely still have to wait until late 2011 to get the last book.

Tor's attitude seems to be that it's more reasonable to get one book every 12 months rather than two very quickly and then a long wait for the third. Plus a summer release for TOWERS would interfere with promotion and publicity for THE WAY OF KINGS. Tor probably wouldn't want two Sanderson books out in rapid succession.

I believe Sanderson also has to write the second WAY OF KINGS book (more properly, the second STORMLIGHT ARCHIVES book) to get it ready for Summer 2011, which means he can't get A MEMORY OF LIGHT done any humanly faster.