Calling a funtion when scripted content is on screen

I am writing a script that shows ads over media (games, videos, etc) on arcades and other sites for a set period of time. The ads go away and the media shows often before the media is scrolled down to. I notice that WidgetBucks ads don't load until they are scrolled down to.

Anyone know how to do this in JavaScript?

 

Using scrollOffset http://www.quirksmode.org/viewport/compatibility.html and onScroll http://www.quirksmode.org/js/events_compinfo.html

That should do it.