The answer to the issue you imply with your comment "so, in other words, no one knows for sure until it happens" is that those of us who believe Mr Camping is wrong are doing so on the basis that we acknowledge that we do not know when Jesus will return. It is perfectly possible it could have been on May 21st 2011, in which case we would all have looked ridiculous! It might very well be tomorrow. It might be one hundred years from now. Mr Camping is not wrong to assert Jesus is returning - all Christians believe that - his error is in stating specifically when. He has no logical, biblical, rational or even inspirational basis for setting a date of May 21st, or any other for that matter. By doing this he not only brings the faith into disrepute, he also wastes huge amounts of people's time, energy and money that could be better spent elsewhere.
Okay, so how are you certain that Jesus will return? How is that different from Campings certainty that he will return on a given date except that his can be tested and yours can't? So, your certainty is really kept alive by uncertainty, isn't it?