All that aside, here is what I do every time someone pitches an idea (no matter who they are and what the idea might be):
1) Listen
2) Think about it (is this unique, good, stupid, wait... don't I already have this app?)
3) Ask them some key questions, (how will it make money? How much time do you have every day to work on this? How much money can you spend on this project? Have you done any research on the idea? What other apps are there like this one?)
4) Pull out the iPhone and do a quick search for obvious app titles matching their idea and show them the results then ask, what about all these? Have you tested all of these?
5) Give them a 30 second lesson on using the internet and the app store to research their ideas. (nicely)
6) Give them my contact info if they don't already have it - even if the idea sucked (remember 90% of your ideas suck too) and why miss an opportunity to network.
7) If the idea sparks an interest, Say hey, Id like to see a layout of that sometime. Or if its really great set up a time for a call.
8) If it blows, don't worry, they will probably have forgotten about it a week later.
9) Congratulate yourself on not being an ass and realize that you helped someone, yourself, and the rest of the developers out there. They will be better prepared the next time they begin the "I have a great idea..." speech.
This only takes 5 mins and you really never know what you might hear. Ideas are great because they make us think, they test us, they inspire us, and sometimes they can make us rich if can figure out how to use them well.