I have had a thought...
correct me if i'm wrong, but so far the only part leaks have been the internals of the 4S yes?
and the N90a we have seen the exterior of.
Now here's a genuinely new thought...what if Tim is going to do the entire keynote with the '4S' in his hand and on display and then FINALLY the 'one more thing' is going to be "but i have got one thing to tell you about the iPhone 4S, it does all this..but it doesn't look like this...it looks like THIS"
Hey presto iPhone 5 reveal.
Crowd go EFFING MENTAL.
I only make this suggestion, for the reason that things just don't add up. iPhone 4 vs iPhone 4S? 90% of the whole reason Apple sold as many iPhone 4's as it did is because it looks new and sexy, and as other people have mentioned it's the fashionista exclusivity thing! If they don't release a new design, why upgrade at all? In whatever guise be it 3GS or 4 the iPhone still does what you need it to do, get your e-mail surf the web, run the apps etc etc.
I would seriously consider keeping my 3GS running for another year if there isn't a redesign, because do I really need iOS 5 and the assistant feature? No...