A lot of people say this will happen, and a lot of people say it won't. Honestly, I don't think it will, but here's a possible scenario that I see.
As of today, Tuesday the 18th of September, 2001, the US is at war. They're not sure with whom yet, but believe me, whoever they are, they're in for a fight, unless they totally give in, in which case they're in for a savage unilateral beating.
Many people with low IQs, little piggy eyes, and an overactive patriotism gland are not ruling out the possibility of the US using nuclear weapons. Many other people, who snacked on lots of wall candy when they were kids are suggesting that the US use nuclear weapons against Afghanistan, without actually bothering to negotiate first. Just blow them up for blowing-them-up's sake.
Reasoning behind this is as flawed as a drug test Donovan Bailey can pass, and goes something like this.
The entire world (literally; votes in the UN Security Council and General Assembly were unanimous for the first time in history) is on the side of the US, except the people who are specifically against the US, like, for example, terrorists. Therefore, the US can do whatever they want.
The entire world is on the side of the US against terrorism. They will not, however, back: 1. Use of nuclear weapons without an alternative (of which there is always one), or 2. The staggering amount of innocent casualties that would result.
There would be no collateral damage from the attack as Afghanistan is in a remote area.
There would be no direct collateral damage to the US as Afghanistan is a country that no non-aid-giving American really cares about. The countries around Iran, however, such as former Soviet republics, China, and Pakistan, all have nuclear capabilities, and Iran, another close neighbour and border-sharer of Afghanistan, is working furiously on developing nuclear capabilities. While these countries may not have obvious ways to deliver warheads, it's quite easy to decimate a city by setting off such a weapon in a harbour.
Also, radioactive fallout would quite possibly kill thousands and injure/disfigure/otherwise affect millions of people who are not, and never have been, Afghans, let alone terrorists.
Afghanistan is our enemy, therefore we should use every weapon at our disposal to strike back.
Those in power (the Taliban) who support terrorist activities within Afghanistan's borders are half-enemies, but the real enemy is the terrorists themselves, and while nuclear weapons would kill people who are terrorists, it would also kill thousands of civilians whose only crime is to be under the heel of an opressive military/religious fundimentalist regime that they are powerless to bring down without outside assistance.
World War III could start in Afghanistan if the US is stupid enough to use extreme force in a situation that does not call for anything less than a very finely tuned, carefully thought out, well-planned strike, but it will not be Afghanistan that starts it, it will be the US.
Here's hoping it goes over better than I expect.