I hear app ideas all the time. I think at least a third of the time I can pull out my phone and find an app that does that already. If they haven't even looked at competitors, they're not serious. I've learned that one quick way to filter people is to ask to see what apps are on their phone. If it's just Facebook and Angry Birds, they're useless.
Out of all the app pitches I've heard, I'd say 33% are already done reasonably well by other apps. Maybe 25% or more are not possible to do on the iPhone (but sometimes on Android). At least 90% have a marketing plan of "just put it in the store and wait for the sales to explode."