As SSJ stated above, there are two major network technologies, CDMA and GSM. Most phones are already made for both technologies. Remember, just because the iPhone is AT&T only in the U.S., different carriers have the exclusivety in other nations, and they probably don't use the same exact network technology.
Look at BlackBerry, they've been available for every network forever. Look at the Motorola Razr, name a company that didn't offer a Razr at some point. Look at every "free with new plan" Nokia or Samsung. Obviously cell phone companies are more than capable of making phones for every and any carrier out there.
Exclusivity agreements are about one thing, profit. When companies consipire to limit consumer freedom for profit, then it becomes a matter for the government. There's no proof that's what is happening here, but that's why the FCC has been asked to look into this.