Nikon refurb stuff turns up from time to time via their dealers. I'm not sure of the situation with Canon, but Nikon returns are not refurbished by Nikon themselves, but by a company called JP Solutions and they come with a JP Solutions warranty, not a Nikon one.
The last two I bought I returned. The first, a D50, has an SD card in it with someone's pics on it and a filthy sensor and the second, a D300 had a knackered AF motor, neither appeared to have had any refurbishment work done other than a wipe down with a damp cloth.
You certainly get a guarantee, but as you're essentially buying someone else's problem and a problem that probably hasn't been sorted, I wouldn't touch another one with a barge pole.