Is it possible to switch iPhone carrier if iPhone isn't available for Verizon soon?

My whole family is on a Verizon plan. I currently have an iPod Touch and love it. If it were a phone, it would be amazing. I wouldn't have to carry around an iPod and a phone. So anyway, I'm getting a phone for my birthday and really want an iPhone. That doesn't mean I'm getting it, but I've been looking. I heard the iPhone and at&t contract is up next year, but does anyone know when? And if I do get an iPhone, is it possible to change the carrier to Verizon?