APC say you should plug their UPSes directly into a USB port on the computer, rather than though a hub.

I'm not sure why... Perhaps it's to ensure you're not plugging it in through an unprotected mains-powered USB hub. Or maybe there's another reason... Try it and see what happens.

I've never been certain why APC says one should plug the UPS directly into the computer instead of a USB hub. I suspect there is some obscure technical reason; but hypothetically there is no reason why you couldn't plug in the UPS via a hub, as long as the hub is itself plugged into the Battery Backup side of the UPS.

I ran tests just last night, pulling the plug on the UPS unit: battery power kicked in instantly and the PowerChute software displayed an alert box a few seconds after that. The software also "gracefully" shut down my computer when the battery reached the limit I had set in the software preferences. I successfully repeated this test several times.

So, it appears that the UPS unit does indeed function just fine through a (powered) USB hub when power is cut; and the PowerChute software also kicks in just fine if you're running it, even though APC says it is not fully compatible (what in particular is not fully compatible about the software, I don't know).

Incidentally, I have a Backups Pro 500, which has my 533 MHz G4, a 19" CRT monitor, an Aiport Extreme Base Station, and a 4-port USB hub plugged in to the Battery Backup side. The UPS's USB cable is plugged into that hub.