Pale said:
Just as a point of correction: The "Great Wheel" has been the default AD&D cosmology since the release of the 1E Dungeon Masters Guide. It's not just canon for Greyhawk and Planescape.
Go back, read the DMG, stop saying false things.
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Just a point of correction to your "correction":
The Great Wheel appeared in Appenidx IV of the PHB (1978), All encounter charts and text references regarding the planes in the DMG (1979) direct the reader back to the PHB.
BTW: This is tangental to the issue, but it should be noted that there is no mention anywhere in the 1E DMG or PHB indicating a connection to the not-yet-published World of Greyhawk (1980), which even in that supplement, was only assumed. All explicit affiliations of the GW to Greyhawk were established elswhere.
Regardless of its origin or specific connections, there are a few vital points at work here:
While the GW was the assumed cosmology of the D&D game pretty much from the onset, it was not compulsory. It was merely one appendix of various optional material, along with psionics, the bard, and suggested treasure division. It is fluff to be used or discarded at will.
Wizards of the Coast has decided that due to numerous mechanical and accesibility reasons, a new, less restrictive cosmology was needed for the assumed backdrop of 4E. One that isn't straight-jacketed by alignment (which is de-emphasized in the new game), and will allow for a wider range of personal tweaking, easier insertion of third party material, or complete abandonment.
Arashi Ravenblade's claim that Wotc is changing canon established over the years is not really correct. No actual canon related to the Great Wheel cosmology has been altered, it has merely been decoupled from the core game, left to stand as it's own campaign universe.
They key is its relationship to other universes:
AD&D Players Handbook 1978 said:
There exist an infinite number of parallel universes and planes of existence in the fantastic "multiverse" of Advanced Dungeons & Dragons.
All WotC has done is make the default setting be a different universe, parallel to the GW/Planescape universe, the Forgotten Realms universe, the Eberron universe, the d20 Modern universe, the Mutants & Masterminds universe, the Gamma World universe, d20 Judge Dredd universe, etc.
This means that a DM who prefers the GW universe can continue on as normal, albeit with some conversion necessary. They could even have players travel to the 4E assumed universe maybe by way of a portal somewhere in Sigil, and have to deal with its alien cosmology... or eschew it entirely, just as they probably have been doing with the Ptolus universe and its alien cosmology.