An interesting question came up on the otn thread ( http://forums.oracle.com/forums/thread.jspa?messageID=9772504�) a couple of days ago. The original poster was asking on how to avoid superfluous updates when using the MERGE command. He said that he wants the update to start only if one of the columns to be matched has been changed; otherwise he would like the update to be ignored by the MERGE command when a matching key has been found.
I included my answer within the thread. How, in your case, would you proceed to accomplish the original poster requirement?