It happens all the time to me and I'm sure to you as well. Someone suggested I implement an iphone app that scrapes the exchange web interface and gives you calendaring, email and all the other goodies because he was stuck logging into a locked down server blah blah blah. All I had to do was code it all up in a weekend and he'd split the money with me.
So lets say I actually looked at the feasibility of it. Now if I go off and implement a revolutionary email program for android and make a billion dollars do I owe him anything?