Welcome to the forums. I live in west central Minnesota as well (McLeod County).
In the SW corner of the state, here is how it works:
If you are a current Verizon or Unicel customer, you will become an AT&T customer and will get a new phone. If you are a current Alltel customer, you will become a Verizon customer and won't need a new phone.
At the moment, AT&T service is pretty poor in this part of the state. I happen to have AT&T because I wanted the iPhone, and I do get reception in the town I live in and in my home. Also I get coverage all the way to the east to the Twin Cities. However if I go much further west, no native AT&T coverage. Currently a person may think they are getting AT&T coverage in the SW part of the state because the phone does say "AT&T", however it is actually roaming, and is not native. Also native data speed in this part of the state is currently only EDGE and can get about 180 kbits/sec. If you are roaming, it is just GPRS that is excruciating slow with a speed of about 4 kbits/sec. So it is about only a 10th of full GPRS speeds.
However that is about to all change. The current Verizon towers in this part of the state are all being fitted with AT&T GSM equipment. Rumor has it that it will be 3G speeds (we can only hope). So one can expect that the coverage from AT&T once all the towers are converted and the "switch" being thrown, should be at least as good as the Verizon coverage was. Actually it should be better, as the towers coverage will be all of the existing AT&T towers plus all of the Verizon towers that are in the process of being converted now. I received a tip a couple days ago that the switch is currently planned to be flipped in little over a month, Oct 24th (subject to change). Also AT&T is opening new AT&T retail stores (or converting the existing Unicel stores) in the cities of Hutchinson, Litchfield, and Willmar.