This is kind of exciting.
First of all, the phones sold by AT&T have been purchased from Apple by AT&T, correct? In that case, if Apple is about to do something big, they have cleared out their inventory and AT&T is responsible to take care of their inventory. Not Apple's responsibility.
April 1st is obviously a sentimental day for Apple. Maybe something big will happen.
Certainly 3G/iPhone 2.0 firmware is married to WWDC. It would be very shocking for them...