This page describes the hardware and software design of a printer power switch controlled over USB from my Linksys NSLU2, aka Slug. The unit can, however, be controlled from any Linux box, and can switch anything, not just printers.
NSLU2 acts mostly as a file and print server. You can go for weeks without printing anything, so if you want to keep the printer switched off when your not using it (it takes about 4W while idle, which must be more than 99% of its total energy consumption). But if your printer is placed a floor away from you, maybe you don’t like going up and down to switch it on and than again to collect the printing. So it might be a good ideea to get a power switch.
There are plenty of power switches out there. Mostly these are expensive things intended for server rooms (see this example) with RS232 or Ethernet interfaces; USB would be more appropriate for the Slug.
Slug-Controlled Power Switch: [Link]