Apple’s favorite son AT&T has changed its upgrade eligibility plans and dates for the iPhone and in doing so it has possibly unveiled to the world the release date for the upcoming iPhone 4G. MobileCrunch is the source of this news which they received from one of their loyal readers. 21st June is likely the date on which iPhone HD will go live.

Here is how the reader explained this huge huge development:

Not sure if anyone [..] has noticed this same thing, but my Upgrade Eligibility date on my AT&T account switched from 11/21 to 6/21 in the last day or so. I bought a 3GS on launch day last year, and my upgrade date read 11/21 since then. A few other folks in my office have noticed the same changes happening to their accounts as well.

Those who follow Apple closely won’t be surprised on hearing this date, since the Cupertinos have always unveiled their ever successful mobile phone in the month of June. If this turns out to be true, this means iPhone 4G won’t be launched at the WWDC10 event which kicks off on 7th June.

As a matter of fact 21st June might be the correct date as Apple have booked Yerba Buena Arts Center for 22nd of June. But here is the thing 21st June is Monday.