An eager, eagle-eyed XDA member spotted a Google ad while searching for "xperia ion" that may be showing AT&T's hand concerning the release date of said device. One of a few ads that appear when searching for the device states that the Xperia Ion is "Coming Exclusively to AT&T in June." Well, that narrows it down a bit!
The ad doesn't give a specific date, but it does narrow down the date range significantly. It's worth noting that, if this is true, this device will be coming out very shortly after the EVO 4G LTE, and maybe even close to the release of the SGSIII, if Sprint doesn't drag its heels too much.