Megaton's unusual immigration laws seem to have created a condition where a substantial portion of the population of the town is in this situation. Essentially, new visitors to the town seem to be approved individually by the Sheriff/Mayor, Lucas Simms. Meeting this approval allows one to remain inside the town's protective walls, but does not constitute citizenship. Non-citizens of the town are not permitted to own housing, and instead must either rent lodging at Moriarty's Saloon, or for the less transient settlers, space in the common house is shared communally. (The PC is, initially, considered to be transient and thus is not welcomed by the other settlers to share the common area in Megaton.)
Upon meeting criteria which are not made exactly clear to the player, settlers may eventually become full citizens of Megaton, many of the settlers will express a wish that "maybe someday they'll let me buy a house here". The Lone Wanderer may become a citizen of Megaton by disarming the bomb in the center of town during the quest The Power of the Atom. It is not known if the Lone Wanderer would be considered a Megaton settler prior to this, as this seems to be more of an informal status.
If your Karma is at the higher "good" levels, once per day (if you are in town), a Megaton settler will come and give you some gift, such as bottle caps (sometimes even only 1 cap), food, a stimpak, or ammunition. The settler can usually be found around the entrance of Megaton, and can vary in clothing, as well as change equipment throughout the game. This may be triggered after completing the quest Galaxy News Radio, maybe as a sign of your deeds and the radio's popularity.
If you decide to destroy Megaton with the bomb located in the town's center, any surviving settlers (known as Megaton refugees) that you encounter will immediately become hostile towards you.