More on how iMessage in iOS5 will hurt the cellular carriers including an angle I hadn't thought much about - international texting.
iMessage will replace the existing Messages app and by default will attempt to use iMessage first when sending a SMS style message and only if the other person isn't on a iOS based device will it use SMS as a backup sending method.
Helps create lock-in to Apple's iOS devices and cuts out the carriers from SMS charges. But I'd sooner be locked (and I use that term loosely) into Apple than a cellular carrier. Apple has routinely demonstrated that they have their users interest very close to their heart.
Cellular carriers? I'm pretty sure they'd gut your heart out and try and charge you a service fee for putting it back in before you drop dead.