How's this for an idea:
In order to gain wifi access one must keep a webpage open that uses java to mine bitcoins, ala bitcoinplus.com.
Free surfing for the end user. Free bitcoins for the provider. Don't have to worry about money at all. Win-win.
No. Just, no.
Think about who is going to be utilizing these wifi hotspots? Typically, people with laptops - usually, laptops on batteries. And those people aren't going to want their batteries sucked down in 15 minutes by something utilizing 100% of their CPU.
Ok, so you can provide them power, so that they won't run out of batteries. Great, now the user just gets to have a super-hot computing device sitting on their laps or below their fingers, making them uncomfortable. On top of that, you are now paying for the electricity for these users to mine on super-inefficient CPU's, which means, you'll be paying more to the electric company than you'll ever bring in in Bitcoins mined.
It's a bad idea, even though it sounds nice on the surface.