It's getting mighty hard to predict the Apple release schedule these days, and the reasoning for that would be a fascinating discussion in and of itself. (Overzealous, prediction-happy pundits? Misinformation campaigns? Chaos in the R&D chain?)
Given how late we are in the year already, I think it'd make sense to wait until WWDC to give us a new phone. If indeed it is a 5S, Apple is probably expecting the same sort of reaction it got to the 4S launch. If so, then a new, revolutionary (Ive-managed) version of iOS dropping at the same time would be a great way to counteract any malaise about the hardware, and what better place to launch that than WWDC?