Apple has posted
Power Mac G5 Uniprocessor Firmware Update 5.1.5f1, an update aimed specifically at Power Mac G5 systems that use only a single CPU. The new firmware update improves general system reliability and restores sleep functionality, according to the documentation included with the update. The download consists of a firmware updater utility that is copied to the Applications/Utilities folder. It requires Mac OS X v10.3.5.