Apple doesn’t like to leave a portion of the year uncontested. Last year, that happened, and we saw the release (and success) of devices like the Galaxy S III during the spring, when Apple had nothing new or shiny to show.
Will that happen again this year? According to recent rumors, Apple may be pursuing a more aggressive schedule than last year. If you will remember back, Apple used to hold a spring event where it showed off the latest version of iOS. They would then ship the first beta to developers, but they wouldn’t show any new hardware. The hardware would be held until the larger event (WWDC).
If Apple is indeed pushing to have the iPhone 5S ready by August, it could mean that they are also planning to host an event in April or so where they show off iOS 7. iOS 7 would ship with the iPhone 5S, as new versions of iOS always have, but would be available for developers to install and test on existing devices.