The portal website at http://www.genie.co.uk, run by BT Cellnet, provides a very useful email <-> SMS gateway for users of mobile phones on any of the British networks. If I want to send an email from my mobile phone I can either go the WAP route, with all the complication and expense detailed by hemos above, or using Genie I can simply:

Compose an SMS message offline, ensuring that the first word is the address of the person I want to mail. Once finished I simply fire off the SMS to phone number 212 and within seconds the recipient is opening mail from {my mobile phone number}@sms.genie.co.uk. No fuss, no bother, it just costs me the price of a standard SMS -- about 10 pence. The recipient can reply to that email address and it will be sent to my phone as an SMS.

OK, so I'm limited to 160 characters, less the length of the email address, but being able to send email from a mobile phone is an extravagance not a necessity so I'll put up with that.