There is a MSG CENTER aka server for this usually your network operator provided. I don't think all the world's network provider will give their concession to Apple user. So if Apple setup their own MSG Center the network can also block them. And they are in USA. Why would one want to send a SMS to USA and then back when the person receiving it may be only few hundred meters away from yourself?
iMessage connection is by 3G or Wifi, there's no way providers like SingTel can block it. Its true that it'll send back to US, but if Apple is confident of launching it, it will delivers. Its the same as Whatsapps idea, but Apple > Whatsapps.
Basically, Apple telling phone providers to fuck themselves. LOL