That's what it is.
There are two numbers. One on the bottom of the 3DS that extends the warranty, and another on a card that comes in the box that registers it for Club Nintendo.
Weird, mine never came with one either.
Did you buy used?
Nope, got mine new for Christmas in 2011. I checked my Club Nintendo account and it said I registered it on December 26th, 2011 but I'm positive I registered with the serial number and not a CN code.