The US military is about to adopt a new doctrine concerning the use of nuclear weapons according to which nuclear weapons could be used preemptively.

From the document (Chapter III):

(1) Geographic combatant commanders may request Presidential approval for use of nuclear weapons for a variety of conditions. Examples include:
(a) An adversary using or intending to use WMD against US, multinational, or alliance forces or civilian populations.
(b) Imminent attack from adversary biological weapons that only effects from nuclear weapons can safely destroy.
(c) Attacks on adversary installations including WMD, deep, hardened bunkers containing chemical or biological weapons or the C2 infrastructure required for the adversary to execute a WMD attack against the United States or its friends and allies.
(d) To counter potentially overwhelming adversary conventional forces, including mobile and area targets (troop concentration).
(e) For rapid and favorable war termination on US terms.
(f) To ensure success of US and multinational operations.
(g) To demonstrate US intent and capability to use nuclear weapons to deter adversary use of WMD.
(h) To respond to adversary-supplied WMD use by surrogates against US and multinational forces or civilian populations.

Also from the document (Chapter I, under e) International reaction):

[...] while the belligerent that initiates nuclear warfare may find itself the target of world condemnation, no customary or conventional international law prohibits nations from employing nuclear weapons in armed conflict.

The Times report on this:
http://www.timesonline.co.uk/article/0,,11069-1776250,00.html.

The document itself: http://www.globalsecurity.org/wmd/library/policy/dod/jp3_12fc2.pdf.

Meanwhile, as Laura Rosen reports, US diplomats have been showing Powerpoint presentations to diplomats from other countries in an attempt to convince them of the reality of a nuclear threat from Iran.