Well, I guess if Apple is planning to sell phones through Verizon as has been rumored for a long time, then turnabout is fair play. The latest scuttlebutt says that AT&T is readying to announce its own next-gen Android-based cellphones sometime next week, putting the news smack dab up against any potential Apple announcements at WWDC. The HTC Aria is the latest device that AT&T is supposedly welcoming to their network, but it could be any number of Android phones, really; if AT&T goes for one, there's no reason they won't invite a few more in as well.
[As noted by commenters, AT&T began selling the Motorola Backflip in March, which is an Android 1.5 device; however, the phone has been reportedly held back from the full Android experience, and cannot install arbitrary applications like other Android devices. The Dell Aero is also announced for the carrier but does not yet appear to be shipping. –Ed.]
Of course, we don't know the details behind all of this. Given that we've never actually seen the agreement between AT&T and Apple, it's possible that AT&T was free to sell any handsets they want at any time,
[Since the Backflip is present in AT&T's lineup along with other smartphones, it's clear that Apple is not preventing the carrier from offering devices of many flavors. –Ed.]