If someone steals your car...
you call the police.
It's a lot trickier in the idea business. (And more and more, that's what we do, right? Invent and propogate ideas).
I'm on record as recommending that you do very little to protect your ideas and move as quickly as you can to spread them. Michael Cader's Publisher's Lunch Deluxe today points to Dan Brown and Lewis Perdue -- Dueling Da Vincis: Legacyan extraordinary indictment of the Da Vinci Code.
What's so neat is that it is entirely possible that Dan Brown never read or was even aware of the Perdue book. That's the thing about ideas... unlike cars, they go many directions at once and are quite nuanced. I bet that the books are actually quite different.
Other than gaping at the charts on this site, I'm not sure what you should do with this tidbit. Maybe just realize that if you've got a neat idea, it's likely to be "stolen" one day (be prepared to live with that) and if you hit a homerun, you're probably going to get sued (sigh.)