Pat Robertson suggested a few weeks ago that the assassination of Hugo Chavez, the president of Venezuela would be an easy way out for the US, much easier than invading a country, like the US did with Iraq. Now, no matter what you might think of Robertson, Chavez or the general usefulness of trying to change the leadership of other countries, I think he has a point. It is just not in the advantage of the US (and by extension the rest of the west).
War is an expensive business, even for a country as rich as the US (let alone for any country that should get invaded). Having a small band of professionals kill the leader of a country you're not so happy with, is much cheaper both in money and in lives for both the invaders and the invadee. Still, the world didn't quite agree with Robertson to the point where he was forced to make his apologies. Now that might have to do a little with the fact that he actually suggested a target and not merely suggested a strategy, but I doubt it. Political assassination as a means of foreign policy is for some reason not done, which isn't that strange if it weren't for the fact that it is less done than invading countries.
There are theories that Saddam Husein had his secret agents plan the assassination of Bush Sr and that this had to do more with the last Gulf War than all the Weapons of Mass Destruction. It certainly seems to be the case that the Americans after the invasion tried to specifically kill Saddam with aimed bunker busting rockets. The Israelis certainly aim for Hamas leaders, so this whole killing of enemy leaders isn't totally strange. But it certainly is frowned upon.
I think that it is partly because it levels the playing field and it lowers the cost of attack. If any country could just reasonably cheap eliminate any foreign leader it didn't like, things would become very chaotic. You sometimes hear that instead of war, the leaders of a country should just play a game of cards and this would make things much more peaceful. Rubbish of course. If say, Liberia could invade the US with a near fifty percent chance of success, they would certainly be tempted. Same with assassinations. It would make war too cheap to be not an option for a president in trouble.
Currently the US is the only country with enough financial resources to afford itself the odd invading of another country. China might get there, but isn't. It is in the advantage therefore of the US to make war more expensive, not cheaper (and arguably by threatening the Soviets to take the cold war to space, the US did make it too expensive for the communists).