According to a recent post at the LiveJournal Nintendo DS community, some malicious Animal Crossing: Wild World players have figured out how to remove your town's gate while visiting via Wi-Fi.
Though it's unclear as of yet how the vandalism is actually done, it seems that what happens is the visitor is able to instruct the host's game to replace the town gate acre (yes, Virginia, Wild World has acres, they're just not obvious) with a museum acre. Once the host saves the game, the damage is done.
Reportedly, the damage can be repaired with the use of an Action Replay MAX and a save-editing tool, but it's probably only a matter of time before vandals think up newer, more destructive things to do to folks. Be careful out there.