(10am edit:added misc notes)
I went to the “Gen Con Design & Development: Presented by D&D Insider” talk, which was run by Rich Baker and Stephen Schubert (with Bill Slavicsek joining for the Q&A session). There were some answers about Essentials as well as other miscellaneous future-plans comments:
Facts about Essentials:
1. Bill Slavicsek confirms that Essentials will not be replacing the PHB 1. They will continue to print the core books as needed. (Slavicsek said the no-reprint rumors were untrue and seemed a little annoyed about the whole rumor.)
2. The monsters in Essentials will be iconic d&d monsters, like goblins, orcs, giants, and trolls, but they won’t be replacing current MM1 monsters, just adding new varieties of those monsters.
3. Essentials won’t have a slew of new feats, since there’s kind of a sense that D&D 4e already has a bit too many feats.
4. Some powers will be updated to make them slightly stronger/weaker with the essentials release. Example: Lightning Bolt will have a new keyword (evocation, which matters to essentials mages) and do half damage on a miss.
5. Essentials will also be helping codify skill challenges, expanding on work already done in the Dungeon Masters Guide 2.
6. Dragon Magazine Support for Essentials: It sounds like there will likely be at least some support specifically for Essentials in Dragon Magazine. And of course since most of the powers between the different builds for each class are shared (such as encounter powers for an Essentials fighter build and an PHB 1 fighter build), an article that adds more powers to a class is going to support all builds of that class (save, say, a daily power for a build in Essentials that doesn’t use dailies). The general idea is that there’s every reason to believe that if someone has a really cool idea for an Essentials specific class feature, there’s as good a chance of it making it into Dragon Magazine as any other awesome idea.
1. WotC is unlikely to be changing all the MM1 monsters to fit with the new power levels in the MM3, but will likely be coming out with some guidelines to adjust your MM1 monsters some time in the future.
2. On Con/Int Races and Martial Controllers: WotC has no pressing desire to try to fill “holes” in certain power type or stat combos since a fundamental part of the design philosophy of 4e is to avoid needless symmetry.