A friend of mine had his 98 Boxster into the shop for just that, speedo calibration. Aftwards he claims it is dead on. I don't really mind if it reads a little high, that way when I think I'm doing over the speed limit I am actually not and have a safety margin.
I have no idea what it cost him to have this done if anything, but I seem to recall that he said it wasn't hard for the dealer to do.