As a Sprint rep, if someone calls with a non-sprint meid, it'll tell me that it's not loaded in the system. It was the same in the glorious days that I had VZW, and I assume it's the same for every other CDMA carrier.
When you have a phone from another carrier, the only places that can get away with activating it would be third party stores, where they would flash the phone. Make sure you get some endorsements for the store (other people stating they had their phone flashed with no problems) and then see if they can flash it.
Again, keep in mind that with non-SIM phones flashing is a risky business, and you can just as easily end up with a 4.3 inch screen, 1ghz processor, HDMI output phone that will only be good to make and receive cal... (continues)