In an answer to another question, a statement is made:
Sharpening is the only (? I can't think of any others ?) edit which needs to be done differently depending on the target medium. E.g. you would sharpen a 20"x10" print differently from a 640x480 pixel image for a web page.
What should be taken into account when sharpening for various output methods? How would one sharpen for a print vs. an onscreen image vs. other uses and how does the final image size affect sharpening?
Edit/added: with printing, does the surface/material of the print (paper, metal, canvas, etc) affect how much one should sharpen?