I have Cingular (just basic GSM) and it works for me fine in Seattle and the Eastside (Redmond, Bellevue, Kirkland) as well as all down the I-5 and I-90 corridors, as well as I-82 (down to Yakima) and into Pasco/Richland (the Tri-Cities).

At Microsoft, Cingular (AT&T) is one of our primary providers (along with Verizon) so most (if not all) of our corporate-issued Smartphones use them for service.