Hmmmm... I've generally seen the question posited the other way around, "what can I use from 3.5e in my C&C games?"
Once the Monster book, the adventures, and the Xagyg campaign material come out, I think the answer to your question becomes more obvious.
As for the rules from the PHB, it's really a different game, so it might just be a case of getting ideas as to how to do a game set in a similar genre very differently. You could add some of the C&C class powers as class specific feats, perhaps adopting the Knight, Assassin, and Illusionist whole hog. I don't know how different the spell lists are, but you could check out how it does some of the same spells to see how they're handled differently. You could try out the less complex combat procedure and see if it meshes with your 3.5e game.
R.A.