Greatwyrm
Been here a while...
The classes in the Player Essentials books do not include all the classes in the PHB, and are variants of the classes that are likely incompatible with previous books still in print such as Martial Power 2.
Have you had a chance to listen to the podcast yet? I'm not sure if it's an Insider Only or not.
Dungeons & Dragons Roleplaying Game Official Home Page - Article (D&D Podcast: Essentials)
Now, I'm no court reporter, so these may not be verbatim quotes:
"We didn't want the game that branches into two games."
"A person can mix and match their Essentials products with their existing D&D books."
"Does this new version of the Cleric replace the one in the Player's Handbook? No... it's much more like a new build."
"It would be like choosing a class feature instead of a new daily power at that level."
"There's less for you to manage, but your character's just as effective."
"The powers that a Domain gives you, you get them, but you don't have to keep them. You can still swap them out if you wish."
"It's no different than if we did a Player's Handbook 4 and said, 'instead of new classes, let's do new twists on the old classes.' Just like the Power books did with the Beastmaster Ranger..."
"...here is a power in a later book, or an earlier one, that I'd like to swap in -- we'll let you do that."
"Once we're done releasing the Essentials books, the books you'll get next year, are just like the books you're getting now."