[wildjim]

That's just slang! You can't actually steal an idea!

Once someone has made it possible for the idea to be shared -- even if they wrote it down and locked it in their safe -- then the information is already potentially accessible!

Breaking-in to someone's home to steal the paper you wrote your idea on is explicitly the theft, and getting the idea written on it is a side-effect.

[tvon]

Looking at the OED definition: http://is.gd/eSaRt

first example:

> dishonestly pass off (another person‘s ideas) as one’s own: accusations that one group had stolen ideas from the other were soon flying