I've had people on other shards copy my house designs. I camped an idoc on europa that was a copy of a house I did during one of the expansion betas. I entered a contest on another messageboard and one of the other entrants copied one of my house designs (that got a little ugly). I entered an EM sponsored halloween contest and half the other people in the contest stole my original deco and used it. Someone was selling a house on Baja and I complimented their house design - it was a copy of one of mine, and I was being tongue in cheek but they brazenly thanked me and didn't admit to the theft.
It's actually kind of funny after a while. I did learn my lesson about the deco contests - keep your house locked up and private till the last day you can enter, especially if you've created some brand new deco. Or it *will* get copied by the shameless.