Free Wi-Fi is on its way to some Japanese vending machines. Working much like a mobile hotspot at your local coffee shop, people located near the machines would be able to connect to the internet for 30 minutes at a time and surf the web.
... and after that 30 minutes, all you have to do is sign in again.
Pretty cool idea, but I think I'd attach it to a stronger revenue model than "hey, this will keep people near the machine and maybe they'll buy something." On the other hand, setting up a way to require purchases by individual users would be a complicated pain.
What I'd do is put a visual gauge on the front of the machine that says "Wi-Fi On: X-Minutes Remaining." No logins, etc. required, anyone can access the Wi-Fi. Any time someone buys something, X goes up. If X reaches zero, Wi-Fi access shuts off -- for everyone -- until something is purchased. Maybe set it so that X never goes higher than 30 minutes, and so that a purchase ups X by 10 minutes. Or whatever numbers work to keep a steady flow of purchases coming.