Don't worry, I'm sure AT&T will filter all your iPhone traffic and count 1 iMessage as 1 SMS message regardless so there's no "confusion" for the end user trying to figure out why their SMS usage has gone down by 80%.

Apple have never shown any interest in being friends with the carriers. Since day one of their foray into the mobile phone business, they've been barely tolerating their partner carriers - like a horrible roommate, you cannot kick out because you need their share of the rent. It's just now Apple feel strong and established enough to completely shun them.