It was actually released as an update on the 12th, the iPhone 4S was released today (14th).
I downloaded an installed on the 12th, and thankfully didn't suffer any of the problems some people experienced with the extremely busy servers timing out. It take 3 hours to do my iPhone 4, but the iPad and the wife's iPhone 4 were a lot quicker.
Quite a nice update:
(a) I like being able to get to the camera from the lock screen, and being able to use the volume + button for the shutter. Incidentally it is possible to use the volume + button on the headset too. So some enterprising people could surely wire things up for long distance remote shutter release.
(b) Notification centre is a huge improvement. When you have a lot of apps using notifications in iOS4, having to clear down dozens of notifications before being able to use the phone for anything was a tad annoying. You couldn't go back and see what notices there were, as once they were dismissed they were gone forever.
(c) Remote syncing and backup over Wifi is great too. When I go to bed, I plug the phone in to recharge, and it looks for my Mac and starts to backup automatically. I don't have to remember to periodically connect the thing to iTunes and back it up.
There's quite a few changes in there, but I'll let people discover some for themselves.
In the meantime iOS5 will tide me over until my contract finishes and I can decide whether to go for an iPhone 4S or hold out longer for an iPhone 5.