My IBM Thinkpad did that to me after I plugged in the cradle for my Nokia 9300 (at least that was the last thing that worked on it)
Same behaviour as you...USB interface hardware was present and working as far as the OS knew..but nothing responded when plugged in to a USB port.
In the end it had to go back to IBM for a new Mainboard under Warranty.
On your machine. have you only tried the front ports ?
There are a few different wiring conventions for the USB header, it's possible that those ports wired on the header are wired incorrectly (it is also possible for this incorrect wiring to shut down the USB interface but usually the OS knows about this)
So I'd try using one of the ports that are directly connected to the mainboard (on the rear I/O shield panel) and preferably also test these with the front panel header disconnected.
The problem you have also sounds a bit like a missing 5 volt USB bus power supply.
If you have a test meter handy then you can sacrifice a spare USB lead by stripping it back and testing for 5volts across the red and black wires inside.