Unfortunately there is no way to track a lost 3DS. It would be a good idea to keep a record of the serial number just in case so you can identify it if necessary though.

I agree and to avoid being stolen just don't pull it out in really public places.

This would be a great way to protect from being stolen.  My 7yr olds 3DS was stolen a few weeks ago by a neighbors son.  Even after the police were called and witnesses that saw him take it were questioned it has not been returned.  Im not only upset my sons DS was stolen but I feel bad for that boy who now has learned that he can steal and get away with it and not be held accountable for his actions.  He was bragging on the bus that he stole it and that his mother knew and was allowing him to keep it since we called the police on them.   

As a friendly reminder, please don't bump old threads. The last time this thread was active was almost a year ago, and the question has been answered.