Nobody keeps a secret like AT&T, and by that I mean nobody keeps a secret quite so poorly. Boy Genius Report and other sites are talking up an internal Death Star memo that asks employees to hold down the fort during the June 15 - July 12 window. Vacations should be moved if possible, says management, and no additional vacations are being approved during that period.
Since AT&T pulled a similar Mr. Burns-esque maneuver last year prior to the iPhone launch, it's entirely reasonable to pin this year's lockdown on the arrival of the 3G iPhone after WWDC. It's probably not necessary to stake out your place in line just yet, unless you happen to be an elected official or have an injured knee from a previous iPhone queue. If you were planning an iPhone purchase in the next four weeks, however, this is some of the strongest evidence yet that you really should put your credit card down and think it through.