The USB spec calls for 500mA/port maximum, although most vendors have been exceeding this for USB hard drives since forever.
I myself have a USB hard drive plugged in through one USB cable, so the port is definitely capable of handling more than 500mA for spin up, but I can't vouch for that being sustained as the hard drive I have is rated for 500mA draw.
As for shutting off the USB ports, I know it is possible (Windows will do it if there's a "Malfunctioning USB device" on the port) however I'm not aware of any facilities in linux, or specifically within the PLX kernel we're using that allows for that functionality.
Best of luck with your Pi! I'm jealous.