My plug in is on the back of my computer which sits on the floor under my desk....so, it's a PITA to have to crawl under the desk to un-plug the cable.
On my XP machine, I have an icon on my desktop that I can enable and disable, although I'm not sure if a hacker could get around that or not and is it really disconnected? (do you know???)
On this new computer I'm setting up (Win 7 Pro) the network connection icon only shows "Enabled" and gives no option for "Disable". The best I can tell is that un-plugging the cable is the only way to take the computer offline
So, I'm thinking the next best thing is a switch that would make it convenient to physically disconnect the internet cable