http://en.wikipedia.org/wiki/Dumping_%28pricing_policy%29
Have I got this right? Since the PS3 is currently being sold below it's cost of production if Sony were to sell it at a lower price than the equivalent model of the 360 (and as long as the 360 is being sold above production costs) Microsoft could file an antidumping petition since it could be deemed as a threat to the industry of the importing country (ie the USA).
I am guessing the laws are alot more complicated than this though.