Apple quietly removed the single processor model of the 1.8 Ghz Power Macintosh G5 today, as pointed out by several reader reports. The removed model still appears on the hardware tech specs page for the PowerMac, but is no longer available for purchase in the online store.
Additionally, reader submissions point to a recent delay in Mac mini shipments of 6-8 business days. While this could be just a delay in product availability, an imminent refresh to the line is not outside the realm of possibility.