Boy Genius has the news that AT&T has blacked out the month of June for employee vacations in their stores. In the past, they've done exactly that when, oh, say, a brand new iPhone launches. So it's possible that we could see a new iPhone out and ready to go even before it's expected in July of this year.
Of course, we know that iPhone OS 4.0 is due out sometime soon, and because the new OS features will only work on the current 3GS model so far, it's not a stretch to think that Apple is prepping a brand new iPhone. A new handset has long been rumored to work on Verizon's network, but this would suggest that even if it is spread across multiple carriers, AT&T will have it for sure.
We'll keep our eyes open as usual, but if you were planning to buy an iPhone and can wait until June for a new model, that's probably a good idea.