In the ongoing saga with certain Windows Phones not playing well when unlocked (specifically MMS issues), there was an official tool and an unofficial one. The former was Samsung's app located in their app store ("Samsung Zone") and it was evidently quite limited in function (going by the average rating of 1 star). Later an unofficial homebrew solution was produced though it required you to have developer access/jailbreak the device.
We then got word that Samsung was working on an official fix, allowing users of unlocked phones to reconfigure their device for other networks with a simple menu selection. That update has hit today (v1.9) and from early reports seems to work quite well, even allowing MMS on new networks. The app also goes pretty far in reminding you that if you are currently in a contract with a carrier this tool will not work, so be warned. Other than that, Samsung seems to be at parity now with HTC and LG in offering network reconfiguration tools for users. For that, we're quite happy.
Grab the program here on your phone via Zune Desktop.
Thanks, Philipp, for the tip and feedback!