Jake i think makes a good point, the only way the US could consider an invasion of Iran would be through the draft, which again i think jake rightly states is not politically viable at present as american opinion is negative towards the iraq position and without another 911 style incident the draft is unthinkable.
in relation to N Korea being intimidated by action in Iran, i think this would only encourage them not discourage to be honest, for the same reasons the iranians feel able to more confidently stand up to the US for the reasons above, the kim jong would be in a far stronger position if america was fighting wars on many fronts because he can bluff to a far higher degree without actually threatening american soil, for the same reasons Iran is now but even more so....to have any bargaining power you need to have a position of strength, it would only be a less stable world if america trys to impose it's will be force alone at every problem, as mr churchill said, jaw jaw is better than war war...
besides if the US decides to invade Iran the NK's will merely have to get in line to launch attacks on the US and it's allies, i can't believe i'm about to use a chuck norris analogy in the middle of a political debate, but minty i see what your saying but i would extend the scenario you painted, if chuck was your landlord and he wasn't busy, sure you may get your butt kicked, but if hes already fighting three other blokes, you are more able to walk up behind him and kick his backside without him seeing you