According to Howard Forum discussion thread here, if you have gotten your hands on an HTC Touch Diamond, then chances you have a Euro edition which translates into bad mobile experience if you are an AT&T customer.
The reason is because this phone (June 2008 release) is designed for Europe. Not only its 3G data is set for Europe (WCDMA 900/2100MHz), its tri-band (900/1800/1900 MHz) voice network is only tri-band and favors T-Mobile USA instead of AT&T (mostly 850MHz, rarely 1900MHz), where as T-Mobile USA is primarily running on 1900MHz.
In layman's term, you should wait until this phone is available (est. Sept 08) for US edition running on WCDMA 850/1900MHz as well as for its voice network. You'd think going by GSM network, life would be easier right?