Here's a little tip: Recently I purchased my HTC Desire, like many of you did, through T-Mobile utilising their 300 texts and 300 minutes with free internet 'booster' tarrif, however this morning my first bill arrived so I promptly registered on their site and checked my tarrif.
Delving through T-Mobile's 'booster' options I found this small print:
Internet for 'always-on' phones (Android™ and Windows® Mobile). If your phone needs an 'always-on' internet connection, internet will be included in your price plan, so you can choose a different Flexible Booster. This will be highlighted on the phone overview page.
I've phoned T-Mobile who told me on their records I had a Nokia N600 (which I don't) and now that they've set me up correctly with an HTC Desire I can indeed use my booster for something else (as internet is included as default). So now I'm getting unlimited texts and internet for £25pcm. Nice.