The most recent major polls show Obama getting anywhere from a 4 point to 20 point bump in approval rating due to the killing of bin Laden. The average bump was about 8%. See here for the results: http://www.realclearpolitics.com/epolls/other/president_obama_job_approval-1044.html
That’s good news for Obama—if it lasts—since his approval rating has been stuck in the 40s for some time, a danger sign for sitting Presidents up for re-election.
Going back to FDR in 1940, every sitting President with an approval rating of 49% or higher at the time of the election won re-election. Every sitting President with an approval rating less than 49% lost. Additionally, if the President's approval rating is in the mid-to upper 50s or higher, he should win the popular vote by 10% or more and thus be pretty much unbeatable.
That's their approval rating at the time of the election or just before. Further out you get from the election date, the less predictive are the approval ratings since a lot can change. At this point in the election cycle, the trend matters. Obama was stuck in the mid-40s and has been there for awhile. The bin Laden bump helps a lot, if it lasts.
Someone made the point that about 70% to 80% of American voters seem to pretty well set in their opinion of Obama. 30% to 40% either really like him or really hate him. His approval rating seems to bottom out in the mid to low 40s since about that % like him regardless of the economy or anything else. In contrast, Carter got down into the 30s. On the other side you have the Tea Party and extreme conservatives that really hate Obama. They are about 30%+. They would hate him regardless of what he does. So we are left with the 20% to 30% in the middle--weak Democrats or Democratic-leaning Independents. So, it is their reaction to events that matters the most.
Interestingly, in the most recent CBS/NYT poll, Obama's approval rating went up among Independents (43% to 51%) and Republicans (9% to 24%). His bump for killing bin Laden (46% to 57%) was larger than the bump GW got for capturing Hussein in 2003 (52% to 58%).