The way I understand it, plain hats are ok, they just can't be branded with any company or organization that does not have a marketing deal in place with the NFL. That rule has actually been around for a coon's age.
I'm actually curious why they haven't been enforcing the 'John Randle Facepaint' rule. He used to paint himself up like the fifth member of KISS and the NFL made a rule against it and started fining him, causing him to quit doing it. Quite a lot of players (including Ray Lewis) have gotten back to doing it, apparently without repercussions.
I always thought it was a stupid rule, but when has that ever stopped the NFL? Maybe it's just 'not a point of emphasis' right now because they're sticking to hats and defenseless receivers. Spearing and facepaint is now ok, though.