Oh no, not at all. When your contract is up, you just go month to month with your service. Your account doesn't close, or service shut off. It continues on as-is.
The nice thing about being out of contract is you can cancel your service without paying the hefty Early Termination Fee.
So, if I closed my account in two years, I would have a device that didn't get 3G or 4G or have a phone number. But I can still use WiFi...I'm assuming. Just like having an ipod with WiFi, no data plan attached.
I want to do this partly in protest of the tiered data plans, and rising rates of all carriers. But mostly to save $70 a month. But I still need email, YouTube, some apps like BOA.