Are you talking refurbs or service replacements? That's two different things. Refurbs are products that were returned, etc. and are repacked and sold on the Apple refurb store. I think (but not 100% sure) that these items retain their original serial number, but they do have their original sale date reset so you get full warranty.
Service replacements are manufactured out of new & used components and get a new serial number. You can't buy a service replacement, they're just used for exchanges in lieu of repair.