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).
yes the problem is they are going to be tightening the noose so to speak regarding the data usage thats happening in the next few months. thats when you wont be able to use IM on the low level accounts. I actually spoke to the [T-Mobile] technical guys today and they listed the days where I hooked my phone to my macbook pro - actually scared me cause I didnt even really batter it [English Translation: use it a great deal], but they knew the periods. the guy said that they can tell when its being used on a laptop / computer and he said that tmobile were going to be streamlining webnwalk to make sure everyone gets good speed and basically people dont "play unfair"