Specifics of my own issue:
new iMac 20 (latest iteration, firmware up-to-date), OSX 10.6.2
2.4 GHz Intel Core 2 Duo Processor
Recently instaled Snow Leopard.
Purchased: WD My Book Studio 1Tb.
Plugged in via Firewire 800, turned power on, recognized by Mac, Time Machine sees the drive...