I have the same problem - I thought initially, it was because I had flashed my Desire with a Generic ROM, but by reading this thread, it would seem to me that's its a problem with the phone itself.
I phoned Orange last week and they instructed me to change the Network Mode (Settings -> Wireless & Networks -> Mobile Networks -> Network Mode) to "GSM Only". I tried this for a few days, and it seemed to work ok (but i didn't receive a lot of calls)....when I switched it back to "GSM / WCDMA auto", the problems returned the next day.
I would urge others to give the above a try to see if it really does solve the problem...
Oh, and remember, it could be worse....the signal could go completely if you hold it in your left hand
BTW - My replacement Desire is arriving this morning! I will let you all know if it solves the problem.