Just so you know, this is a very small industry -- like 2 degrees of separation. Anyone who is working in the field knows better than to steal someone else's work. Doing that would black-list them forever. If someone does steal your work, it's only doing them harm. Sooner or later, you'll hear about it and you can prove that person wrong.
It's happened to me before. Someone put one of my old pieces in their portfolio and claimed it as his own work. I found out about it through one of the guy's co-workers who had somehow seen the piece before. The co-worker did some searching online and found that particular piece in my web gallery. He emailed me about the situation and asked if I could provide proof that I am the original artist. I did, and the guy who stole my work got fired on the spot.
As Kal said, I wouldn't worry about it too much.