They've concluded the world has no stomach for a war against them.
The world does not. The question is whether or not President Bush and the USA have the stomach for it.
The US is pinned down in Iraq at the moment. This is no doubt the cause of the rhetoric and actions coming out of Iran. To go after Iran at this time, the US will have to get on a full war footing with a military draft. We will need many more troops. The Middle East isn't the only region with tyrants looking for opportunities to engage in mischief.
Bingo. We have a winner. Exactly. Iran is calculating that after Iraq the U.S. is spent and neutered and they know the U.N. won't do anything.