I suspect that the changes to the PHB will be significantly less than folks expect. Errata will be applied, clarifying text inserted (hopefully better combat examples and illustrations) and a smattering of new material and implemented material from the classbooks (particularly feats that have appeared multiple times). 34 pages is not a lot of space to add new material with, and I expect some art changes, clarifications, and not much more. Personally, it would be nice to see the bard and ranger changed, to make them more interesting. Both get most of their class abilities at first level, and never really get more interesting. The ranger, the 'one level wonder' that many take one level of because of 'front-loading', is another problem. I expect many rules will become more optional in this edition, such as the Paladin and Monk's multiclass restrictions.
The DMG will probably have incorporated new rules mechanics (such as the underwater rules from Dragon), a mass combat variant, a few new prestige and NPC classes, and perhaps some sections on how to create and balance prestige classes...and, of course, more magic swag. I would expect charts to appear what they're actually supposed to be near. Probably some additions on gametype alternatives (such as more emphasis on xp for non-combat encounters) and more DM advice.
MM will probably incoroporate the monster creation article from Dragon, new information (various ACs, ECLs, updated errata for all creatures) and some new beasties, too. They may choose to format it differently, not just new art, to better appease one of the largest criticisms of the book.
I would pay full price just for errata-ed books, let alone new content. I doubt older versions will become radically out of print, any more than they did prior to this, when WOTC released the classbooks. I suspect if they do make sweeping changes to things like classes, that there'll be web-enhancements released with a summary of those changes, for the disenfranchised.
I personally think its much ado about nothing.
