This is desparate rhetoric from a regime that will eventually be ousted (looking like sooner rather than later).
It becomes serious when the Iranian army starts blantantly and openly crossing borders and killing American/Israeli soldiers; and/or detonating a nuke somewhere. Until then...its a bluff. Its a bluff even more so because the Iranian regime ain't dumb...they know their country would be a glistening crater of glass if they plan on using WMD's against American soldiers or Israel proper.
BTW...I hope I'm right.
They may not be dumb...but the fact remains that these are sheep willing to blow themselves up for their idiotic beliefs. I believe that these guys, if they get nukes..WILL use them. Unless the people of that country can overthrow that regime, we are looking at big time trouble.