UPDATED: Comparing the before and after states of carrier.plist, iambored8907 reports in the comments that the update added PNG graphics for AT&T's new MicroCell (Femtocell) service:
ORIGINAL STORY: Apple today is pushing down a "Carrier Update" via iTunes. As usual, explanations are scant to none. Have you gotten it yet? What carrier are you on, and are you noticing any differences?
(Thanks to Earless Puppy for the head's up!)