I have switched from Orange to T-Mobile and went for the Desire at the same time. Oddly enough, neither Orange or T-Mobile are great at my home (can't get HSDPA and only get 2-3 bars on the GSM signal) but T-Mobile is very strong at my work.
T-Mobile's 3GB Internet allowance for Android phones (although you will have to fight for this with customer services of you don't get your phone from T-Mobile directly) and flexible boosters are very compelling. From reviews of mobile data services I have read T-Mobile and Vodafone have the best performance. Vodafone probably has the best network from what I can see, but they are more expensive on the whole. I see too many flaws with 3 to go with them, while a friend quite high up in the industry told me that O2's network, especially in central London, is rather creaky, especially for mobile data because they had sold so many iPhones when it was exclusive to them. I left Orange because I despaired of the customer service (after over 10 years of being with them too) and their mobile data options and quality of service were not very good.
I like what I have experienced with T-Mobile so far, although occasionally I do come across uninterested customer support people. On the other hand some of them have been absolutely brilliant. Do consider that T-Mobile charge for voice mail (it won't come out of your bundled minutes) but I worked around this by signing up to HulloMail.
Ian
PS I got my Desire from mobiles.co.uk (part of Carphone Warehouse) partly because they were competitive (although not the absolute cheapest), they had stock, and the phones they supply are unbranded and unlocked.