ahhhh.....ok....
"Do Not Seek Praise. Seek Criticism."
"If You Can't Solve A Problem, It's Because You're Playing By The Rules."
"The Perosn Who Doesn't Make Mistakes Is Unlikely To Make Anything."
-Paul Arden
I used to spend a lot of time on a sports car forum, and they blacked out their plates for another reason that I don't see mentioned so far: to prevent law enforcement from tracking down owners who post pictures of illegal activities (street racing/burnout competitions, etc.). A lot of guys would post videos on YouTube or elsewhere, so when they posted still pics of their car they wanted to make sure the license plate couldn't tie the two together.
As SomeBeach and others in this thread have suggested, I don't know whether the law has time or even cares to look at these forums, but in places like SoCal where street racing is a serious issue, guys just didn't want to take a chance.
I suppose a law enforcement officer could use the tag to track down someone who is racing but the actual racing event could not be prosecuted based on a video on youtube. The officer would not be able to testify to the time or place. He could also not prove who was operating the vehicle. Then throw in all the other things cops need to address and it's unlikely it would spark much interest.