Much like Jim, I have a mind that soaks up numbers. I do have my smaller carry pistol's number memorized... easily done because the serial number is matched on one of the magazines for it, so I see it all the time at the range when reloading and it's stuck in my head.
I've only read the serial on my regular carry gun 2 or 3 times, and it's not stamped in large enough numbers anywhere on the gun to make it conspicuous enough to see unless you're looking for it. I can only remember the first 3 numbers of it offhand right now. Now I'm gonna have to actually go look at it so I'll remember it.
EDIT: Just read the serial. It's now in the ol' memory banks. It's an easy one... a visually-appealing (to me) 6-digit number. My memory for numbers is visual... something about the image of the number and where it is on the object is what sticks in my head. It's the same with passages in books. If it's something I remember, generally I can tell you which page in relation to the binding (left or right) it's on, and what area of the page it's located. Yeah, I'm a nerd.
I can't remember what gun's I have stored where much less their #'s, I have to record them.