I’m feeling that way, too. I’m scared for him, and I feel deep down that he can be one of the greatest presidents ever.
Hell, I already voted for him, and I watched the infomercial anyway! 
If Obama is elected (big if…don’t want to count any chickens, etc) he will probably be the most hated president (in lunatic fringe quarters) since FDR. But I take solace in the fact (at least, I firmly believe it to be true) that the Secret Service is incredibly good at what they do.
The risk is extremely high, relatively, for any president to be assasinated. There’s been, what, 42 presidents, and how many have been shot at? its like 1 in 5! Don’t worry about Obama,any more than any other president. There will always be thousands of people in the world who would want to do it. I don’t think it’s any more likely to happen to Obama than it would McCain, or was for Clinton or Bush either.
Lincoln, Kennedy, Theodore Roosevelt, Ronald Reagan, Garfield & McKinley are the 6 I can think of and many many more attempts.
One guy tried to hijack an airliner to kill Nixon. Ford was shot at by several Manson Family nutjobs. A crazy ninja-wannabe hopped the White House fence during the Carter years; Jimmy was also stalked during the 1980 campaign by the guy who later shot Reagan. A suicidal guy flew a light airplane into the southwest facade of the White House early in Clinton’s first term. Somebody threw a dud grenade at Dubya while he gave a speech during a trip abroad a few years ago. And so on and on.
Still, for the reasons I stated in post 59, I’m afraid that Obama is especially at risk. I’d like nothing more than to be proven wrong.
You forgot this rather notable attempt on Jimmy Carter’s life.