I don't see why they couldn't make plans (like a phone) and add calling and texting to future handhelds.
Hopefully, people will do all of their calling, messaging, etc. through programs like Skype and Facebook in the future anyways.
(The PS Vita can run those programs.)
Also, it seems like a longshot at the moment but if all of those features could be put into one program with live updates to your device, any mobile device with internet access could be using a universal messaging and calling system.
Edited by Daniel, 21 April 2012 - 05:54 PM.