Its imposibble to having they back (For execption for Rolf)they will leave your town forever, in Animal Crossing: Wild World, if you befriend a villager, they will give you picture of themselves so you won't forget they forever, the picture are rare and can't be found anywhere else.
NOte: Rolf will move to your town if you talk to many times if you visit your friend town with Wiiconnect24/DS wierless communication or WFC, if he wants leave your friend town, talk to him in many times, he will sometimes move back to your town, hope I helped :)
