This is a ping entirely different both visually and audibly from the typical Alert Ping which let's the players immediately know to withdraw from the pinged area.
Here is a short video (13 seconds) that I made showing both types of pings in LoL, for those that have never played it:
http://www.youtube.com/watch?feature...&v=yzqXZd3kM9Y
I know, everyone hates everything about LoL but I actually feel this was one of LoL's few "useful" additions to the genre.