iPhone OS History:
March 6th, 2008 - Apple demos and releases a beta of iOS 2.0 and a new SDK to developers (no mention of a new phone)
June 9th, 2008 - Apple announces the new iPhone
July 11th, 2008 - Apple releases iOS 2.0 GM to the first iPhone along side the iPhone 3G.
March 17th, 2009 - Apple demos and releases a beta of iOS 3.0 with a new SDK to developers (no mention of a new phone)
June 8th, 2009 - Apple announces the new iPhone
June 17th, 2009 - Apple releases iOS 3.0 GM to the first iPhone and iPhone 3G
June 19th, 2009 - Apple releases the iPhone 3GS running iOS 3.0
April 8th, 2010 - Apple demos and releases a beta of iOS 4.0 and a new SDK to developers (no mention of a new phone)
June 7th, 2001 - Apple announces the new iPhone
June 21st, 2010 - Apple releases iOS 4.0 GM to the iPhone 3G and iPhone 3GS
June 24th, 2010 - Apple releases the iPhone 4 running iOS 4.0
I predict that this year will be the same. iOS 5.0 beta and a new SDK at WWDC with another event to show off the final version of iOS 5 and announce the new iPhone at the end of August/ early September.