I was recently discussing 6th edition with Big D and then later that very same evening I saw a post by Fritz over at the fritz40k.com discussing when we might see all the codices and a new edition of 40k. Now he seems to believe that we won't see 6th edition for quite a long while. You can check out his predictions here.
I, however, have some very different predictions:
April 2011: Grey Knights
October 2011: Necrons
February 2012: Witch Hunters (It will be a marine variant inline with the every/other marine codex cash cow)
July 2012: Warhammer 40k 6th Edition
September 2012: New Edition Release Codex ( Wouldn't a very popular army - Chaos make a great codex for a new game release, also immediately following the release of witch hunters?)
December 2012: Codex Space Marines - If you don't think GW will redo their bestselling codex at the beginning of a new addition, you might be just a little crazy.
Leaving Tau, Orks, Eldar and Daemons to not receive an update until early cycle during 6th edition.
Why do I think this? GW has moved to a 4 year cycle on their two core games. It gives them a chance to re-release their games and it is also a more dependable cycle of income. Just like the dependability of every other book being marines, you are now probably going to see the game redone every four years. It will keep the systems 'new' and fresh, and the income moving in a more predictable cyclical fashion. Predictability and cyclical income is a good thing for a business.
So, now I believe, comes the time of speculation - just what changes will we see in 6E? I think it will be more of a 5.5 with tweaks and slight changes. I believe we are still a long ways out from another overarching change to the game like when 4th went to 5th.