According to the Associated Press:
Business social network LinkedIn said it is investigating reports that more than six million passwords have been stolen and leaked onto the Internet.
Although LinkedIn did not confirm if any user data had been hacked or leaked, researchers at U.K. Web security company Sophos say they have confirmed that a file posted online does contain, in part, LinkedIn passwords “hashes.” That’s a way of encrypting or storing passwords in a different form.
Easy enough to fix; go to LinkedIn and change your password. You might think about changing other passwords at the same time; good business practice dictates good, quality passwords changed on a regular basis.