Apple has announced a concept called iMessage (via TechCrunch). It will probably be very handy for Apple users as it is integrated well with the existing messaging system, but apparently it works only between Apple devices. With others you can get the same services and maybe more with Skype. And you could already use Skype with other Apple users. This of course means that you need some kind of data plan, but how many iPhone owners there are with no data plan? If there are some, what they are doing with it?
I can certainly see why companies are producing platform dependent services, but it's still frustrating and one of the main reasons why I don't have any Apple products.