Since a lot of people seems to think that Apple will announce an iPhone 5S this summer (sounds like WWDC to me), I have to say that at the time being, it sounds like they won't announce it.
it's pretty simple, actually. Apple ever introduce a new iPhone without having a X.0 version of iOS in advanced beta testing (2 months or more). For this reason, of they don't announce iOS 7 in the next two weeks (which sounds unlikely), no new iPhone will be shown at WWDC or in June. July? August? We'll see that in May but if they don't show it in May (which would be logical since WWDC will be right around the corner and it would be a better idea for them to announce everything at WWDC), it will be like the last 2 iPhones and it will be announced in September.