Apple has already announced what it intends to talk about at next weeks WWDC. Amongst other things, Steve himself will definitely unveil iOS5. Rumours about what this operating system refresh will bring have been all over the web for months. Improved notifications, speech recognition, support for theiCloud - all these things are more have been heavily speculated on. What is clear, if only by the version number, is this is a major new version. iCloud support is almost certain, as it would make no sense to launch the service (the iCloud has also been confirmed for the WWDC by Apple) and not support your most important devices.
Secondly a few weeks ago mobile phone blogger Eldar Murtazin stated, in this tweet, that the iPhone 3GS will not support iOS5 (strictly speaking he said 'iPhone 3Gs' - could be plural of 3G or could be all in that range. Most of the web seem confident he meant to include 3GS). Eldar is not always correct (though this story has just resurfaced) in his predictions, but he does carrysome weight in the mobile world and has had some notable scoops.
Thirdly rumours have also been rife on the web of a iPhone 4S, abeefed up but not radically different iPhone 4. Some people think a 'stopgap' iPhone this year, with the all singing all dancing iPhone 5 next year. Others think the iPhone 4S will be this year, but much later. Others think we will just see a full iPhone 5 next year.
My argument is this. If Apple does launch an iOS5 that is incompatible with the 3GS, they are cutting off a lot of their existing market. Whilst the iPhone 4 is outselling older iPhones considerably, the 3GS (and to a much lesser extent 3G) still has a huge market share. To unveil your new OS and tell most of your customers you can't run it makes no sense. Unless.. and here it is.. they offer a new phone for those users to upgrade to. The new phone being the iPhone 4S, a beefed up iPhone 4.
A iPhone 4S is the perfect item to unveil in one of Steves famous 'one more thing' moments. An iPhone 5 would surely require its own announcement. But this way the WWDC can be all about the software, with the great news of a new iPhone right at the end. Something like - "It's an iPhone 4 with a faster chip, more RAM, bigger camera. Perfect for iOS5, perfect for 3GS owners wanting to upgrade to the new OS".
This way Apple can launch its new OS next week, give everybody a new phone to upgrade to (and discount the existing 4 for people who don't the expense), and let the whole market catchup to iOS5.
Then we might see an iPhone 5 early next year... well let's leave that prediction for another day!