Well the Iraqi high command would want to keep it a secret probably because if the Iraqi people knew that the brutal dictator was dead, why the people would probably riot in the streets (out of joy) and rip them to pieces. Hell, the Iraqi high command probably fears the U.S. military less than they fear their own people once they find out that Saddam is gone. So they continue to attempt to pretend Saddam is still alive so that they can surrender directly to U.S. troops and come under their protection.
The U.S. would want to keep this a secret because they also want to keep civil unrest in Baghdad to a minimum until they can secure the area. As well, they would prefer to negotiate a surrender with the Iraqi high command so that they can take them into custody and extract important information from them - such as where the WMD are located and other such information.
Anyway, those are the reasons I came up with regarding why both sides might want to keep a lid on this for now (if Saddam is indeed dead, as I believe that he may well be.)