Have you used a task killer to disable anything you are not sure about?
you could try recalibrating the sensor?

 

DO Not kill anything you're unsure of. You could end up with a phone that can't reboot!

If on gsm only, that is the likely culprit, as you can't call and use data st the same time unless on 3g/h

 

There's a proximity sensor on the Desire? I'm a new Desire owner and didn't know that, what does it do exactly?

 

It switches off the screen to presumably save on power - when the touchscreen gets close to your cheek...

 

It's not to save power. It's to stop your ear/cheek accidents pressing on-screen buttons during a call
The exact problem the OP is having

 

Of course the purpose is to prevent accidental touches on the screen. Aaand it would be great if it could be applied to the whole system in general! Eg. when biking and the phone's in pocket facing inwards, the leg is operating the phone right after calls and events...

 

I thought the screen turning off during a call and back on when you've finished was achieved by the G-Sensor. The screen goes off for me when I'm using the loudspeaker and the phone is away from my face, either in my hand or on a flat surface, so I can't see how a proximity feature would work under these circumstances.

 

Now that would be funny (G-sensor). But it makes no sense that the screen stays off when not in proximity. Do you have a Desire??

 

Maybe it depends on how you hold your phone (at what angle) The proximity sensor is at the top left edge of the screen (in the border), under where the power button is - to the left most corner. You can test this by making a call and hovering over that area.

 

Do you have a case covering the proximity sensor?

 

No.

I hadn't even noticed the prox sensor in the corner of the screen until today, although I did think it was clever that it turned the screen off when on a call and then back on when I moved the phone to see what I was doing to turn it off.

 

Might want to have a look at this:
http://androidforums.com/htc-desire...r-disable-screen-while-pocket.html#post930969

 

Update:
http://androidforums.com/htc-desire...-disable-screen-while-pocket.html#post1041850