Three use crossover technology with other networks so have to pay if you use data services of their 3g only network. For this reason they actively monitor device usage and if they discover you're using data on GPRS or Edge (which you will be if you're using a device other than theirs, cos their internet devices will only connect when you have 3g coverage) then they will block it as it loses them money. You can bypass this by getting a cheap THREE phone with si card and tethering, or by buying a miwi.