Just for the info of those reading this, I've been using the basic T-Mobile Web'n'Walk (£7.50*) since October 2006, in a bluetooth phone - with both my laptop and my N800 (and 770 before that). T-mobile haven't complained at all... so I think that if you're sensible with the 3 X-Series you probably won't have issues either. * - Note, you also have to pay an additional £7.50 / month for their most basic voice package (known as 'SIM Only') which gives 50 voice minutes, which I don't use at all. So comes to £15.00 / month.