In case you missed it, Osama bin Laden was killed on 1 May 2011 (which is, incidentally, International Labour Day).
I agree that it is a good thing to have him out of the picture, and that it is unlikely that he could've been removed from the world scene any other way. I believe the killing was justified, appropriate, and worth doing. That said, I am uncomfortable with celebrating his death.
I am uncomfortable with celebrating killing in general. I am prepared to accept his death as positive, on both grounds of preventing more harm and as part of some notion of justice/acceptable-vengeance. By Osama's acts, he stepped far beyond the pale and became an enemy of humanity (hostis humani generis). However, he was a human being, and his death is part of the tragedy of his life. There are times in life when killing is necessary (from the easily acceptable like this to the most ugly necessity like the extermination of the Romanovs). Yet, life is precious, and in each death we lose a lifetime of experiences and bring pain to everyone who loved or liked the person. Contrary to those who believe in an afterlife, we (seculars) understand death as final, irreversible, and a full ending to a life story. Ending that story violently is a last resort, and it is solemn.
We do an ugly thing when we dehumanise another enough to celebrate their killing. No matter their crimes, and regardless of the necessity of the death, we owe fellow people, companions in life who understand pain and struggle and love, respect enough to let them die in peace. We grant this not because they will always do the same, nor because they subscribe to a notion of justice we find recognisable, but rather because of who we are.