"Core" has never meant anything with regards to what I allow into a game or not, and
4E doesn't change that.
On the other hand, "everything is core" seems to mean that I can expect classes from sources outside the PHB I to get support from supplemental materials on a regular basis, and I'm all for that. I hated the fact that many of the best classes from
3E and 3.5E never got any real support simply because they were released in later supplements, while Wizards and Clerics had new spells and feats in every single book simply because they were in the PHB.
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