On the Layer palette select your Background Layer and right click. Select Duplicate Layer.
With this new layer highlighted select Filter / Other / High Pass. Set the Radius to 10 and click OK.
Zoom into your image to Actual Pixels level so you can better see what you're going to do next.
Go back to the Layer Palette and select Hard Light from the left drop down.
Now go to the Opacity Slider and select a level of sharpening that seems best to you. Usually something between 20% and 70% will be best. [/i]
You can also play with the radius for a wide degree of granularity.
I found the guide above here: http://www.luminous-landscape.com/tutorial...harpening.shtml
This brings up an unrelated question (PLEASE don't stop the discussion of the original topic). This technique suggests that you zoom into the actual pixel level. In CS4, Photoshop adds something that looks like a "grid" over the pixels. I find this grid very distracting. I'm sure that there is a way to turn this off, but I haven't found it as of yet.