We couldn't pull off much sense we have a tough crowd...but we did do a little bit. One of my staff modified vB a bit to display the following message to any member who logged in after 12:00am April 1st
The image was only shown once per member and ended with a link to
this thread. We also set up a redirect of 10 seconds in case somebody didn't see the link, they were sure to reach the April Fools message. After a member had already seen it once, he/she would not see it again.
Worked perfectly...we also coordinated it with our monthly newsletter so we had even more people log into the forums that day than average