MPP wants to have the option of the appearance of the front page changing slightly when people visit. Rotating title banners and whatnot. The subject has been covered pretty thouroughly on A List Apart. All I'm doing is making the script run at load time.
The default duration for the cookie to keep the style looking the same is one day.
The nice thing about using an onunload listener is it will also notice if they change to an alternate stylesheet using the browser. (View → Page Style)