It's a metaphor. When "rich man" is mentioned, it means a man rich in sin, or the disfavor of God.

Jesus' comments follows right after he had asked a wealthy young man to sell all that he had, give it to the poor, and then follow him.  It was recorded that when the wealthy young man turned away, Jesus was quite sad because he loved him.  But the wealthy young man had just finished telling Jesus that he had kept all the commandments from birth (which is quite an audacious thing to say).  So Jesus should him by his question that he had not.  For he loved his materialism, his wealth, more than his relationship with God.

Jesus then turns to his disciples and makes this comment.  The comment itself has caused quite a bit of commentary.  Some say that it means exactly what it means, that Jesus is talking about a camel trying to go through the eye of a physical needle.  Others say that the word translated as camel is the same word as the word for "coarse thread" but is missing one accent mark, which would make it difficult but not impossible.  Another explanation is that there was a small gate in the wall of Jerusalem called "the eye of the needle" which camels had to get down and crawl in order to fit through.  But there is some doubt about the existence of the gate.

The most common (and for me reasonable) commentary is that the camel was one of the largest animal in Israel at the time and the eye of the needle was one of the smallest openings.  So Jesus was using an analogy to illustrate the difficulty for a rich person to not love his money more than his God.

But the conversation doesn't stop there.  When his disciples heard this they were amazed and asked, "Who then can be saved?"  For in there culture, wealth was a sign of blessing and poverty was a sign of God's displeasure.  Jesus replied that what is impossible for man is not impossible with God.  In other words, God would make a way for all who wanted to enter into the Kingdom of Heaven.

Just a side note, we know that wealth alone is not a disclaimer because of people mentioned in the Bible who were very wealthy and had an intimate relationship with God.

That's my two cents on the passage. 
 

Great comment.  It is true that those without money can fixate on it as much or more as those who have it.  I agree with the observation.

But when you have little or nothing, it is easier to trust in "a higher power".  When you are self-sufficient (or believe that money makes you so), it is hard to trust anything else but your money.  I have often found that those who are "poorer" are often more generous (proportionately) than those who are "wealthier".