Just a heads up on this for any T-Mob users. Data usage is now going to be capped to 500mb.
So Whats Changing? - From 1st February 2011 we will be aligning our fair use policies so our mobile internet service will have fair use of 500MB.
http://support.t-mobile.co.uk/help-and-support/index?page=home&cat=DATA_CHANGES
http://support.t-mobile.co.uk/discussions/index?page=forums&topic=8010380bff24f8012d6da8429f007cff
Yes, I'm sure it's in their T&C that they can change it, still poor form on their part.

