Well I'm not holding my breath in anticipation of this. It sounds like it could take a few months to roll out nationwide or at least to Belfast. Looking at the t-mobile forums you'd expect NI to get this first. Post goes something like this: "you coverage map shows I would get moderate 3g, yet your CS says there is no 3G in Belfast. Which is it?" Response: "map is just indicative etc". User "why not fix the map!?".
From my own experience I know that their coverage over here is poor but it looks like a few people have been miss-sold on the promise of 3g data.

I'd be more optimistic about this venture. There are a lot of t-mobile customers who will quickly move onto a smartphone (with the promise of data) who could get very disappointed if they are to rely on the t-mobile 3g network.