Yes we went to see it Sunday. "300" has a very simple story (300 Spartans defending their homeland vs. the entire Persian empire), very little plot and ton of fighting scenes. Excellent for a hangover Sunday afternoon :-)
To the Iranian government: yes it is cultural warfare (in a way) but not because of the reasons you propose. Hollywood has been spreading American culture throughout the world for decades by introducing billions across the globe to their stories and values through films. 300 is no more warfare than everyone's old fav "90210"... which was a #1 TV show in Iran by the way!
(300, 90210, 23,... what's with all the numbers today?? ;-)