Why hasn't anyone tried to assassinate Bush?

I was watching something on the Biography channel, or maybe it was Discovery Channel. Anyway, it occurred to me that no one has attempted to assassinate George Bush - at least as far as we know. Now, I am by no means suggesting someone should (the CIA or FBI would be up my ass in a second). It's just surprising to me. He has been in office for nearly 8 years now and for at least half of that, he has to be one of the most hated people on the planet, right? I mean, he has got to be in a lot of people's top five. Even in the U.S., a good chunk of the population blames him for a lot of bad shit going on (rightfully so, in my opinion). More significant, however, is that outside the U.S., there is such venomous hatred of George Bush, I bet he would come in a close second behind Osama bin Laden for most hated man on earth. (a worldwide poll would be interesting) Yet, with all this vitriol and animosity directed toward him, no on has tried to take him out! They got Kennedy and they went after Reagan and Ford and those guys didn't have nearly the "fan base" as Bush. Just strikes me as odd....