I made two aliases in bash for that, like this:
$ alias eth+='sbin/,/ifup eth0' and
$ alias eth-='sbin/./ifdown eth0'
as user i can type eth+ or eth- in the terminal to start/stop internet without that I have to be root, so perhaps I made them as root, don't remember
you can connect these two commands without doubt to a launcher, or, way funnier, to a real graphical switch on your desktop (hint: gtk2-perl is easy for this).