Don't believe everything you hear. Large corporations are about as likely to take your idea and run with it as an Aircraft carrier doing a 180.
Why? Momentum.
Momentum means ability to take a new idea on, think about it and execute. It also means being able to communicate the idea to Learned leaders then asking permission. So by the time it gets there your most likely to get someone with a business degree [0] making a snap decision on some hypothetical tech idea. The other thing is most large corporations area of expertise is probably not hacking (MS, IBM, SAP all have hackers but management is still the problem).
Also telling someone about an idea is pretty lame because ideas are worth, well nothing. But a demonstration of and idea that works? Demonstrating something will spur others to do something ... maybe throw large offers your way.
I did read a post here some time ago of someone mentioning an idea (to a trusted friend) who then created a coy, hired outsourced coders. So demo first, shout second. Shout loud.
[0] Phil Greenspun mentions this in FOW, Ch24, P238 where the head of Wal-Mart, Kevin Turner had but one business degree. This doesn't mean he's not smart but it takes years to train in tech. Is one general are of expertise enough?
-cough-
IBM.
So if people tell you that's nuts or that you are too late or that it would never work, then you might be on to something. In that case you don't need to worry about any one copying it as it will be absolutely unique to you.