Skype wants to change that, and is pushing the FCC to open cellular networks up for non-sanctioned hardware and software. While the benefits of this are clear, allowing someone to use any phone on any network they choose, Skype's motives are anything but philanthropy. They have developed their VoIP software a step further, and want to be able to open it up to users of cellphones. Skype on a cell? That's their aim, since currently many cell service providers block or prevent via contract use of Skype software.
It's an interesting concept and could lead to an interesting battle. While cellular networks are relatively new, they have evolved and propagated much faster than the original phone system. It wouldn't surprise me in the least to see the FCC intervene, at least in the U.S.