HTC Touch Diamond is definitely one of the best Windows Mobile smartphones on the market, but one of its greatest drawbacks is that it is not a global device. As it is only tri-band GSM and dual-band UMTS phone (currently only the European version is released), its usability here in the States is not what it should be, and the reception is weak. However, an unofficial flash of the software (the radio) turns the Diamond into quad-band GSM; 3G support for the U.S. is still missing, but the reception problems in GSM should disappear. This is possible as actually the phone does support all four bands, but for some reason HTC locks the 850 MHz not to be used. If you own one however, we guess you would like to get your software updated and enjoy the increased number of reception bars.
Head over to the thread at the XDA-developers forum and make sure you are downloading a correct radio version (it should be marked as quad-band). If you are not sure what you are doing, please read the same thread; a number of users report that it is really doing the 850 magic.