There must already be existing systems that perform this function, as there 
are many places that are selling metered wireless connections. KCI Airport 
has a system in place, as do many airports across the country, as well as 
Starbucks and other chains.  Some of this is likely being done with open 
source.

I'm curious about a slightly different application foof this concept - is 
there a way to log a user out for inactivity?  There is a "logout 
screensaver" for Windows that will force a logoff if the user is inactive for 
a specified time, but I've never come across the option in an obvious place 
in Linux.