I wouldn't be surprised at 2008 being the next time for an edition. We have the books on the classes, specialized monster books, books on different environments, books on the deities, campaign settings, campaign setting regions, multiple monster manuals, and many more published by WotC. Actually, I could even see them pushing for 2006, it just makes sense from a business point of view. There's not a ton more that they can do that doesn't have to be a new idea, the market for d20 is more competitive than from 2nd to 3rd, and they seem to be the leaders as many products are based 3rd edition thanks to OGL. So why not make a new edition, and then put out all the "new" class books, monster books, etc?