I'm on T-Mobile also, and only recommended those that could be upgraded at Carphonewarehouse. Their phones are all unlocked to any network, so basically you just choose the model that is within your upgrade allowance, and Robert is your mother's brother.
If you wait a few weeks, the value of the Desire will come down quite alot, as they (Carphone and all other mobile retailers) will want to move onto selling the latest and greatest, leaving the Desire as a end-of-line bargain. Thats the other option.
this is true, i have had at look at cpw and the offers are better there as well, as little as £20 pm for unlimited internet access, so i might just wait till the new year for the desire.