QI was stunned to find out that someone I work closely with took an idea of mine and passed it off as his own. I'm not sure what to do -- call him on it or let it go?
AIdea theft doesn't need to be tolerated, but careful steps are necessary to ensure you maintain your own professionalism when dealing with it.
The inner game
This is an emotional issue, and you likely have a range of feelings from volatile ones like anger or passive ones like helplessness and betrayal. Either way, get your feelings under control so that you neither overreact nor give up.
Now, consider your past experience with this co-worker. Does being "stunned" imply that this behavior seems out of character, or are you just surprised that he was this blatant? If there's a pattern, even a subtle one, you'll need to set up more safeguards in the future.
Reflect on your interactions regarding this idea. Could he see it as building on a small nugget of a concept and transforming it? Or did he take credit for a full-fledged idea? Also consider how big a deal it was -- was it a minor insight or a groundbreaking new direction? Do you have documentation or witnesses that establish your contribution, if needed?
Finally, assess the credibility of your source -- do the motives or quality of information of the individual who told you about the situation stand up to scrutiny?
These lines of inquiry are not intended to suggest that you weren't shortchanged on credit for a good idea. However, accusing someone of idea theft, even obliquely, is serious, and you need to be on solid ground if you want to talk to him about it, or raise it with your boss or others in authority.