With the release of this week's IAEA report on Iran's nuclear program, it seems that there is now definitive proof that the Islamic Republic is
That being said, the offensive use of a nuclear device would be an unlikely move on the part of Tehran.
The overall strategic culture of the Iranian government (or both governments if you are feeling cynical) seems to stress the use of hard power only in the defense of the Republic while utilizing soft power and proxies for less immediate threats and more offensive actions. Iran knows that while it has had moderate success jump starting its domestic military production capabilities it does not have the hardware of its potential rivals and as such seeks to avoid traditional military actions, knowing that it can not hope to engage these forces in any type of prolonged assault. Instead, Iran relies heavily on an approach consisting of low cost asymmetrical forces and proclamations of its military might to create a deterrent factor that has thus far kept its borders secure. The former prizes irregular forces such as the Basij militia and a guerrilla-style naval tactics while the later can be seen in Iran's protestations of the past 48 hours in which it is repeatedly warning Israel and Western forces of the dangers associated with attacking the Republic.
Within this model, a nuclear capable Iran would have more to gain from the threat of using a device than it would have from actually using it. Both of the Republic's militaries have demonstrated a very well honed sense of proportionality and restraint when it comes to utilizing their forces and exposing troops to actual combat. They know, despite the proclamations of their government, that they do not have the military or logistical capabilities to survive an extended conflict with a 21st century military and they also know that would be the response to the use of a nuclear weapon on the part of Tehran.
In my humble opinion. the true dangers of a nuclear Iran revolve around questions of regime stability and the amount of security that it can provide for its armaments. There is a growing political divide within the country as well as increasing evidence that seems to point less centralized control of some elements of the military than has previously been thought. This, of course, is likely to be something of a moot point as the amount of Western interest in a nuclear Iran bodes badly for its prospects of being able it see any program through to completion.