If you are waiting for iPhone 5, now you have a solid target day: Apple's Worldwide Developers Conference will start on June 6. The iPhone 3G, 3GS and iPhone 4 were all launched at previous WWDCs.
In the words of Phil Schiller:
At this year's conference we are going to unveil the future of iOS and Mac OS. If you are an iOS or Mac OS X software developer, this is the event that you do not want to miss.
Obviously, the future of iOS and Mac OS X are iOS 5 and Lion. Typically, major versions of iOS would be previewed before WWDC, in March. This year, however, Apple hasn't presented iOS 5 yet. There's still plenty of time until June, but Schiller's words and Apple's invite seem to point out at WWDC as the launch pad for the new iPhone and iPad operating system.
This may mean that the iOS 5 public release could come months later, during the Australian spring. Does this mean that iPhone 5 itself would be delayed? It would be very weird for Apple to break their yearly hardware release cycles, pushing iPhone 5 a few months down the line. Especially with all the fierce competition from Android handset manufacturers - Apple can't afford to slip on that race. However, it doesn't make much sense to show iPhone 5 without a matching iOS 5 operating system.
WWDC2011 will include "more than 100 technical sessions presented by Apple engineers on a wide range of technology-specific topics for developing, deploying and integrating the latest iOS and Mac OS technologies", with over 1000 Apple engineers available for helping developers.