This Niall Ferguson essay - essentially arguing that a failure to pre-empt Iran's nuclear ambitions will set the stage for a nuclear war in the near future - is both well-done and frighteningly plausible.
But it's worth remembering that there has never been real war between nuclear powers. The closest we've come to is the occasional shelling and raiding between Pakistan and India. (Hmmm....maybe China and the USSR, but I'm not sure China had nukes then or at least not more than a few). In any case, here's what seems to me a much more likely scenario:
The US draws down its forces in Iraq, beginning in 2006 and substantially completed by 2008. (Either we will be successful and will be able to draw down or the continuing instability will be exploited by the Kos wing of the Democratic Party to gain electoral success and force the withdrawal). If Iran's nukes are not pre-empted (and is there anyone who doesn't think the Iranians are trying to develop nuclear weapons?), then the Iranians will have achieved a strategic standoff with Israel. But I think they're still unlikely to initiate a nuclear exchange with Israel, simply because the Israelis have enough nukes to obliterate Iran (and, most importantly, its leadership). Rather, Iran will use the nukes as a way of making itself invulnerable to American and Israeli pressure and will then seek to establish itself as the *the* power in the Middle East. This means, first of all, exporting its Islamism to Iraq and Afghanistan, undermining their relatively pro-American regimes. Second, it means undermining the secular regimes in Syria, Lebanon, Pakistan, and Saudi Arabia and attempting to establish a pan-Islamic confederation that both controls a significant portion of the world's oil supplies and, with Iranian and Pakistani nukes, remains relatively invulnerable to international pressure. (The Europeans can't impose sanctions because they are too dependent on the oil and the US will be unable to move against the Iranians because the Europeans - and perhaps the Israelis - will not want to risk the obliteration of one of their cities).
What the nuclear arming of Iran threatens is not a hot war ala WWII, but another Cold War where a radical ideology backed up by the gun takes over a strategically crucial part of the world. Israel might end up as a new West Berlin, hemmed in by its enemies. Not a happy scenario.