Sounds like Apple may be having some difficulty with the high-end G5 machines, and has revised shipping estimates to the end of September. The 1.6GHz units are apparently being shipped readily, but the 1.8 and dual 2GHz models (with the faster frontside bus) are only rarely being seen in showrooms as demo units.
Lots of disgruntled customers… some who ordered within hours of the announcement were originally promised delivery by the end of August. Now they wait another month. Oh, the agony!