Well, aside from PR claims I think the Creation Engine is such a vast improvement on the one used for Oblivion, FO3 and FONV that it's a lot more than just a Gamebryo with changes, given the amount of performance, optimization, detail, features, and so on. We'll know for certain later on though when the Creation Kit is out though and modders such as myself go digging through it.
EDIT: Also agreed with Matt, 64-bit exe's may be hard but the simpler edits shouldn't have to be done by third parties. With Oblivion I understand, next gen stuff, new tech, rushed game, but Skyrim is a whole lot different to me in that department. Nonetheless I haven't had performance issues at all. Not everyone has my kind of rig though..