Jimmy Carter recently penned an op/ed in the Washington Post, some segments:

During the past five years the United States has abandoned many of the nuclear arms control agreements negotiated since the administration of Dwight Eisenhower. This change in policies has sent uncertain signals to other countries, including North Korea and Iran, and may encourage technologically capable nations to choose the nuclear option. The proposed nuclear deal with India is just one more step in opening a Pandora's box of nuclear proliferation.
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Knowing for more than three decades of Indian leaders' nuclear ambitions, I and all other presidents included them in a consistent policy: no sales of civilian nuclear technology or uncontrolled fuel to any country that refused to sign the NPT.

There was some fanfare in announcing that India plans to import eight nuclear reactors by 2012, and that U.S. companies might win two of those reactor contracts, but this is a minuscule benefit compared with the potential costs. India may be a special case, but reasonable restraints are necessary. The five original nuclear powers have all stopped producing fissile material for weapons, and India should make the same pledge to cap its stockpile of nuclear bomb ingredients. Instead, the proposal for India would allow enough fissile material for as many as 50 weapons a year, far exceeding what is believed to be its current capacity.
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There is no doubt that condoning avoidance of the NPT encourages the spread of nuclear weaponry. Japan, Brazil, Indonesia, South Africa, Argentina and many other technologically advanced nations have chosen to abide by the NPT to gain access to foreign nuclear technology. Why should they adhere to self-restraint if India rejects the same terms? At the same time, Israel's uncontrolled and unmonitored weapons status entices neighboring leaders in Iran, Syria, Turkey, Saudi Arabia, Egypt and other states to seek such armaments, for status or potential use. The world has observed that among the "axis of evil," nonnuclear Iraq was invaded and a perhaps more threatening North Korea has not been attacked.

In short, if your packing heat via nukes, you won't get invaded. If you lack the threat of nukes, you're vulnerable to attack.

Given the three "axis of evil" countries, two big reasons for invasion are 1) oil, and 2) no nukes. Iraq has both, oil and no nukes, thus invade. Iran has #1 but has nukes, thus too dangerous to invade. North Korea doesn't have #1 but does have nukes -- a no-brainer, absolutely don't invade.

As Carter implies, other countries will learn that having nukes will keep them invasion-free and after all, if India is given the nodding OK to violate NPT, why should others abide?

Way to go GW! Another idiotic decision to your credit.