The great thing about phone billing is that since the subscriber already has a contractual relationship with his or her phone company, you do not need to enter into one - they effectively act as collection agents on your behalf. The not-so-great thing is that they take a hefty % for so doing.
Since handheld devices are effectively mini computers and getting more and more powerful, it looks like the mobile interweb has a great future
My money is on location based services
