- 2007 June: iPhone - iPhone OS1
- 2008 July: iPhone 3G - iPhone OS2
- 2009 June: iPhone 3GS - iPhone OS3
- 2010 June: iPhone 4 - iOS4
- 2011 October: iPhone 4S - iOS5
- 2012 October: iPhone 6 - iOS6
The list above also shows a neat pattern in the OS numbering, which the iPhone 4S puts out of sync, as it will be running iOS5. As you will remember, there was never an iPhone 2 either, so there is precedent to leave out a number. It therefore makes sense that next year Apple will launch an iPhone 6, running iOS6.
Of course, there is another possibility, and that is if Apple includes LTE next year. Then they may call the new phone the iPhone 4G!
Thanks for reading!