Here’s some Internet settings which you can use on your T-Mobile HTC Desire contract to improve your internet connection in case it’s a bit flaky. Before you start to edit them I’d strongly recommend you write down the original settings in case you want to revert (or if someone is kind enough to post them in comments then even better!)
The settings are as follows:
name= T-mobile Internet
username = t-mobile
password= leave it as it is (or set to blank)
mms proxy= 22.214.171.124
mms port= 8080
To edit your internet settings go on Settings > Wireless & Network Settings > Mobile Networks > Access Point Names > T-Mobile (or whatever your selected option is) and then enter the above settings in the boxes as they appear.
Update: Please remember this is for T-Mobile UK.
Update 2: Credits given to correct person. Sorry sgtbarton!