I haven't played as much risk as most here. But what is the likelyhood of this happening.
I'm playing a 3 man game, and at the end of my 3rd round, I have complete control of Europe, Africa, and S. America, with strong armies along the boarders.
Here's a link. If you click on it and it's no longer the case, it means rounds have taken place since I posted this.
http://www.conquerclub.com/game.php?game=46521
Maybe it's not that big of a deal, but I think, even in a 3 player game, that it's pretty hard to pull off.