>>1
Assassination of heads of state is not a useful political tool. Part of this stems from the fact that killing leaders can lead to martyrdom, and usually merely creates a vacuum for the next evil scumfuck to take power. It does not address root causes. Also, there has throughout history been an unwritten law against assassination of state leaders; they all agree not to assassinate eachother (or at least not without masking the source extremely well).
The US can't (legally) assassinate political leaders. Gerald Ford signed an executive order back in the 70s prohibiting it. He felt, as did most of the Presidents after him, that it's simply counterproductive, especially when an attempt fails (see: Castro).
In the case of Saddam, his assassination would simply have put his even-more-insane sons in power. If by some miracle we got his sons at the same time, Iraq would have turned into what it is right now--only without even a semblance of a stable government.
In the case of Ahmedinijad, his assassination would not change Iranian policy (Ayatollah Khamenei makes policy, NOT Ahmedinijad), and it would provide even more proof to the fundamentalist nuts that the USA is the great satan.