In beta 13 accounting is not implemented yet, though you can get some tracking functionality of bandwidth usage from the bandwidthd feature found under captive portal. that along with DHCP and static DHCP bindings should ensure some validity of the data, allthough a MAC address can be spoofed.
also in the captive portal under graphics, you can see how many people are currently connected, and how long they have been so, though in graph form.
/lone