A great American did indeed say the "obituary" quote, but it wasn't Mark Twain.
As with that MLK Jr. quote from earlier, in which a woman on Facebook's words were inaccurately latched onto a real King passage, a saying falsely attributed to author Mark Twain has been burning across Twitter feeds around the country in an attempt to give context to Sunday's Osama bin Laden killing. The quote, "I've never wished a man dead, but I have read some obituaries with great pleasure," actually belongs to the late civil rights lawyer Clarence Darrow. Darrow's words, in full, are:
All men have an emotion to kill; when they strongly dislike some one they involuntarily wish he was dead. I have never killed any one, but I have read some obituary notices with great satisfaction.
All of these wrong quotes speeding across the internet remind us of another quote, one correctly attributed to Edward R. Murrow:
The speed of communications is wondrous to behold. It is also true that speed can multiply the distribution of information that we know to be untrue.
Incidentally, the phenomenon of quotations being mistakenly attributed to Twain is pretty common.