"How long before iChat (notably missing from the iPhone to date) starts to compete with SMS?", I asked. 16 months, it turns out.
My completely atypical mobile-geek multiple-device-owning lifestyle has thrown an abnormal amount of light onto iMessage, which is clearly Apple starting to replace text messaging in a characteristically subtle way. I'm actually using a Galaxy S2 day-to-day, but I have an iPhone 4 kicking about which I upgraded to iOS 5 the other day. As part of the upgrade process, I had to insert my SIM card, and it appears that during the process Apple recorded my mobile number and associated it with that device.
The upshot is that anyone sending me a message from their iOS 5 device now punts me (without realising it) an iMessage rather than the SMS they would previously sent. iOS seems to detect if you're sending a message to someone else Apple thinks owns an iOS device, and does this automagically. They also downplay the fact that this is going on; the only reason I'm noticing this is that the iMessage goes to the iPhone which sits at my bedside being The Worlds Most Pretentious Alarm Clock, and my iPhone-owning friends think I've stopped talking to them.
It appears that you can't choose to send a text message, but have to deactivate iMessage completely in your settings to get around this. I'm actually a little impressed by how Apple are doing it: it's entirely possible to not notice that they've replaced a core part of mobile communications. It's a little annoying in my case, but I can see that asking which of umpteen different ways (SMS, MMS, Email, etc) I'd like to send a given contact a message, every time I message them, would just be irritating… and, I note, exactly the kind of thing most other device manufacturers have done for years.