Here is a Ruby script that uses sg3_utils to automatically put a SCSI device into standby mode after a configurable idle time period. This also works with USB storage devices (tested with Linux Kernel 2.6.14, sg3_utils 1.11 on Gentoo Linux with an NEC PCI USB 2.0 controller and an external Samsung SP2514N hard disk in an ICY Box).

You can download version 0.1 here.