Isn't it maddening to mention an idea at a meeting, have it dismissed, then welcomed wholeheartedly when another member of the team (usually a man!) brings it up – often at the very same meeting! I do not believe in getting confrontational the first time, as it may be an oversight, but think that you should mention it to the person as an aside after the meeting – explaining that you would be happy to help advise, since it had been your idea originally.
However, if that gets you nowhere or if you feel the "idea – steal" was intentional, may I offer what I once heard as a great alternative? As someone is stealing your credit, simply announce to the offender with a smile "I'm so glad to hear you come back to my original idea from earlier and expand on that". Have you ever had to tackle someone on stealing your credit? How did you do it?