Because we all know spokesman are reliable? If I recall they were denying the Droid prior to it's release as well. It's all about sales, they want them, but they know the iphone won't be coming quite yet.
My personal belief on the matter is september is when it'll be released. Mostly because that's when Daring Fireball speculated, and he's usually a lot more on top of these rumors then 'Thestreet'. And you know there's also that inner child in me that doesn't want to wait so that's also why I believe lol.
But I think if they're going to announce it WWDC seems like the perfect time. Especially if they want to do a late summer release, get the pre-orders rolling, and the hype up. My reasoning for this is also because Apple seems to stick to certain release schedules.
Summer - iPhone's
Fall - iPod's
Winter - (now iPad I suppose)
Spring - Macbooks/iMac
but you know, it being the one last thing at the end of the ipod key note in september wouldn't be that surprising either I suppose.