Just looked at the training guide recently posted and the port does indeed provide 500mA of power (@ 5V = 2.5 Watts), so make sure your usb mass storage devices don't draw more than that. This rules out 3.5" desktop sized usb hard drives and many 2.5" drives.
FYI, iPods use 1.8" drives or flash memory and all current 1.8" drives are under 2.5 Watts.
Here's a great (and brand new) review of portable usb drives:
http://www.tomshardware.com/2008/02/...age_redefined/