If this post appears, it's a pretty good indicator that the "May 21, 2011" Judgment Day prediction advanced by Mr. Harold Camping was the latest in a long line of incorrect guesses about when Jesus Christ would return.
Jesus said on several occasions that He would return at an hour we do not expect.* See Matthew 24-25 for a few examples. He also made it clear that we cannot know the day nor the hour of His return: [The disciples] asked him, "Lord, will you at this time restore the kingdom to Israel?" He said to them, "It is not for you to know times or seasons which the Father has fixed by his own authority." (Acts 1:6-7)
Anyone claiming to have such knowledge is claiming knowledge beyond that which was given to the Apostles, knowledge that Jesus Himself appears to have disavowed in His human nature. In Catholic terms, they're claiming to have an additional revelation that surpasses the deposit of faith which God placed in the Church.
* The trick to prevent Jesus from ever returning is to always be expecting Him to return, thus preventing Him from returning at an hour you do not expect. But I'm sure we have better things to do with our time!