In CS4 it's quite simple to do. Go to Image -> Canvas Size (Alt-Ctrl-C on the PC). You can expand the canvas by a set number of pixels or by a percentage. As an example put a tick in the relative box and expand the image size by 10%. You select the canvas extension colour too. Make sure the anchor point is central (as shown) otherwise the canvas extension will not be evenly spread around the image. You could add another colour around the first by doing it again with different colours and/or percentages.
