This past weekend, I actually saw one of these billboards in the San Francisco Bay Area. It didn't announce that Jesus was coming on the sign, but it did include the date and the website (that makes the claim).
It is amazing that people are citing a particular day -- when not even the Son knows that day or hour (Matthew 24:36).
But, like Greg said, today can be the "day" that people have their very own meeting with the Lord. After all, TODAY is the day of salvation. We aren't promised to live through the end of the day. In fact, many, many people will die today without having ever known the Lord.
As we drove by the sign, my wife pondered, "Do you think those people will become 'post-trib' after May 21, 2011?" I said that I was a bit more worried about the reproach that such loud proclamations can bring upon the Body of Christ and the true Gospel to the world when they make such bold claims.
We don't really need to know just WHEN our Lord will return. After all, as Greg pointed out, many people are dying without Christ each and every day. It shouldn't take a calculation of the end, an expected doomsday, earthquake or tsunami to get us to tell others about Christ. We should be just as diligent as these with bringing the Good News of Jesus Christ to a world where people may meet their maker at any moment.