This was one I had up my sleeve for awhile - to be quietly taken over by a corporate sponsor. Days leading up to the joke, I complained that we were having trouble affording the cost of the website, especially with the fees charged by eZboard to run our message board. That much was true oddly enough, but instead of forcing us to sell out, it merely prompted us to change to a new forum provider. And we thought that it would be good to tie the joke in with an actual website upgrade.
The website portion of the joke was fairly simple. A very drab and "corporate" looking splash page was thrown up that was designed to appear as though corporate was really behind the matter. There were a few extra pages included just for grins, such as a product recall notice showing a kid getting zapped from a toy repair kit, but other than that it was played very serious.
The next day our forum came back online along with the website, except instead of being the "Chuck E. Chat Fan Forum" it was our new vBulletin forum which we still use today. Somewhere along the line I wised up and realized that for as much work as these jokes take, it would be nice to have them make a more lasting impact on the website.