Sean,

If your asking if you can chat to people on their mobile from your computer then the answer is YES, you already can... infact i am doing so as i type! You can use a "softphone" to make calls from you PC. Like this one from counterpath: http://xten.com

If your asking if you can call people on mobile phones using a VoIP connetion then the answer is YES, you already can.

However if your asking if people can use VoIP technology FROM their mobile phone then the answer is a little different.

When it boils down to it, all you need to make VoIP calls is a broadband internet connection. You can actually connect to the internet via GPRS at the moment, im not sure of the speeds, but i think its cutting it a bit fine on bandwidth.

You have to note that even though it may be possible to make VoIP calls from a mobile at the moment, the main stream mobile operators are going to do everything in their power to stop this happening. Shooting themselves in the foot a little you see. Why pay more money to make calls via your actual mobile provider, when you can do it for half the price via VoIP.