This is a nice exercise in what happens when you give free stuff away.
Hint: People try to get the free stuff.
I do think it's a nice idea, I'm not saying you shouldn't do it.
Yeah, I shoulda anticipated problems when Bitcoins went from 0.005 USD each to 0.06 each. If it takes somebody two minutes to go through the "get a new IP, get a new BC address, solve the captcha" process then they'd make 5*30=150 bitcoins an hour, which is $9 USD an hour, which, if you're unemployed, bored, and/or 13 years old is easy money.Hint: People try to get the free stuff.
I do think it's a nice idea, I'm not saying you shouldn't do it.