Your best bet is billing the perons service provider as rtm mentioned.
If you use some sort of password/user system it will be as easy as sharing a password to rip you off. Since mobile IP addresses change every time a person disconnects from GPRS (unless they are the type to leave it connected all the time) & most people delete any cookies set on thier phone as soon as GPRS is disconnected there is no reliable way to keep track of things yourself.
Look into processes of billing telecomunications companies for a full answer
btw, ALMOST any html page can be accessed VIA WAP, some pages are just too big for some phones to handle. I forget what it is called but the pages specially designed for phones are refered to as "cards" & each application consists of a "deck".
Somthing reallyneat, look into Google & WAP

if you have unlimited GPRS on your phone it's a *must have* bookmark
Somthing else neat, If your phone supports JPG, GIF ect... & you have webspace ANYWHERE free or not, resize an image to your phones wallpaper size pop it on that site and navigate your phones browser to the address, most phones will tell you an image has been recieved and ask if you want to save it to your library

Strong with this one, the sudo is.