I'd be careful saying "isolated" - USB ground is computer ground (usually mains
ground too), and will be shared in the hub too as USB signalling is not transformer
isolated (in contrast to ethernet). A powered hub just replaces +5V with a beefier
supply to avoid multiple 0.5A USB devices from overloading the computer's USB port.
The hub power supply may be implemented as isolated in order to allow for slightly different
mains grounds between computer and hub, but once you connect a USB cable between
computer and hub they are sharing ground. But a hub supply probably doesn't have to
be isolated - I wouldn't make such an assumption.
However there are USB hubs available which are fully isolated and use opto-isolated
data signals to allow it to work - they are meant for industrial environments where
large mains-ground discrepancies occur between equipment on multi-phase high current
power networks.