jackwizard from the skype forums wrote this script, not me. I'm
archiving it here because it's useful and Skype's forums aren't
permanent.

http://forum.skype.com/index.php?showtopic=332401

Here's his original readme:

    This script will let you run only one instance of skype,
    rather than opening a new one every time you click the launcher.

    If you've tried to use skype with Gnome-Do you've probably noticed
    that there's no way to bring the skype window back up from Do
    after it's gone to the system tray. This script fixes that quite
    nicely.

    [rant] Why would skype make multi-instance the default??? Doesn't
    make any sense to me, since the normal user expects to get back to
    the running instance when they click the launcher again... [/rant]

    Just download this script, extract it, and put it somewhere on
    your PATH. If you don't know what that means, then just extract it
    to the Desktop and use this command in the terminal:

    sudo cp ~/Desktop/skype-single-instance /usr/local/bin/

    Then you can change your launcher properties to use the
    skype-single-instance command instead of the normal skype command.
    If you're using Gnome-Do: for some reason it doesn't read off of
    the main menu entries... just type the extra 's' (that is, type
    'skypes' instead of just 'skype'), and that should bring up
    skype-single-instance.