Right now IMHO for US surfers it really isnt a viable billing solution (and realistically wont be for another 5 years) however, if you have a lot of global traffic then that is where i would (and have) implement SMS / MMS / WAP billing solutions.
The only real downside to it is that a lot of cell phones are now 'pay as you go' and most of them have high SMS message charges to start off with so adding 'extra' fees to an SMS message may be counter productive in addition the fact that it is my understanding that the surfer actually has to read and/or reply to the SMS message.
But, nothing ventured, nothing gained as they say
