Preview at WWDC likely in June, followed by September launch.
iOS 7 to Ship 'On Time' as OS X Engineers Pitch In
A report from AllThingsD earlier today said that Apple was in a repeat of the scenario from 2007 wherein the company pulled engineers off the development of OS X Leopard to work on the first iPhone.
Apple sources told ATD that it's "not as much of a fire drill" and that iOS 7 "will ship on time". Apple hasn't actually announced a ship date for iOS 7, so "on time" is subjective at this point. However, Bloomberg said earlier today that the public launch of the software was on schedule for "as soon as September".