Is it possible that the Rapture will take place on May 21, 2011? Yes. But it's just as likely to take place on May 20th, May 19th, May 22nd or even 5 minutes from now.
The guy making this prediction is taking the passage from 2 Peter 3:8 literally:
But do not forget this one thing, dear friends: With the Lord a day is like a thousand years, and a thousand years are like a day.
However, the verse doesn't say "a day is a thousand years," it says "a day is LIKE a thousand years." So it's a metaphor and not meant to be an exact time conversion scale.
Again, I'm not going to say it won't happen on May 21st, but the odds of it being that day are no different that the odds of it being today, tomorrow or 10 years from now.