I've had my 2005 Toyota Prius since new, it's been garaged the whole time, and it's nowhere near salt air, yet the car is rusting out at the bottom sill of the hatch door. It seems ironic that a simple panel has rusted, whilst the hybrid battery is still under warranty. Am I expecting too much? Was rust a common complaint in this model?
Rust wasn't a particular issue with the Prius any more than it is on any other Toyota. I would show it to your dealer and have Toyota inspect it with a view to repairing it.