What is Snow Leopard's behaviour when there is a power failure and it is connected to a UPS, and it is in sleep mode? Will the UPS alarm notification wake OSX?
Any change on the USB bus should trigger a wake from sleep, so that the system can see what has happened... (For example, place your system to sleep, and then plug in a USB mouse. Your system should wake up...)
So, yes, as long as your UPS will announce a state change, it should be detected.