Here's something I've been wondering for a while:

I have a USB scanner.  It gets its power from an independent wall wart,
and it has no power switch.  It does go into a powered-down mode when
the computer is off, though, and I can make it do that by unplugging the
USB cable.  Apparently the scanner, even though it is independently powered,
watches the +5V on the USB cable and powers down when that drops.

I would like to be able to keep the scanner powered down the vast majority
of the time when I am not using it at all.  I have heard just enough about
the inner workings of USB to wonder if it's possible on some or all USB hubs
to command down the +5V on a particular port through software.  Whether
there's driver support is another question - right now I'm just curious if
anyone who knows USB better than I do thinks the idea is even possible.

-Chap