Over 450 Nintendo Network accounts were hacked this month, and had their details posted online in a list. This potentially can lead to people stealing your information and using it to pose as you or steal money from you.
The hack allegedly occurred through miiverse, and while the full extent is not known itt is suggested that you take precautions, such as changing your password or watching your bills. A list of the accounts (without sensitive information) was posted here, and you can check to see if you were one of the accounts affected.
If you have been hacked, You should contact Nintendo at once and see if they can help you. You should still change passwords just to be safe even if you were not on the list.