An article from the San Francisco Chronicle (originally from the Associated Press) indicates that security experts and law enforcement officials are worried about a coordinated web defacement campaign set for July 6.
The symptoms so far include a web site (since taken down by the web service provider) and a series of chats that were observed by officials and security consultants.
Probably the most compelling evidence so far is the relative decrease in number of attacks and increase in the number of probes happening over the past couple of weeks. Some experts theorize that this is a sign that hackers are scouting out places to defame on the 6th.
As of yet, nobody is blaming Osama or Saddam.