Last week the U.S. Copyright Office issued a ruling that states cell phone owners are allowed to break their software locks on their devices. There has been quite bit of talk about this on the internet, but the ruling doesn't state anything about requiring wireless carriers to unlock your phone. It just states that people who do so won't be subject to the prohibition against circumventing access controls during the next three years.
The specific mobile phone ruling is exemption number 5 in a group of six exemptions approved by the Librarian of Congress and as you can see in the full rulemaking text the exemptions deal with technological measures that control access to copyrighted works. The specific text of exemption 5 states that people will not be subject to the prohibition against circumventing access controls as it applies to:
Computer programs in the form of firmware that enable wireless telephone handsets to connect to a wireless telephone communication network, when circumvention is accomplished for the sole purpose of lawfully connecting to a wireless telephone communication network.
I usually only purchase SIM unlocked phone from overseas because the latest and greatest mobile tech gear is generally released outside the U.S. first while wireless carriers and the FCC take device manufacturers through the ringer issuing approvals. I don't think wireless carriers are going to start advertising that their phone can be used on competing networks and they most likely will keep on issuing locked phones. You could purchase unlocking services online or if you have been with your carrier and your account is in good standing you should be able to get a free unlock code from them. If you don't intent to travel overseas or switch carriers, then there is no reason to worry about unlocking your mobile phone.