I just bought a refurbished 15" early 2011 MBP from Applestore. I'm assuming that refurbished means that someone had an issue with this unit that Apple didn't want to repair for the customer, but accepted it as a return. With the large number of refurbished units that are available through Apple, I'm thinking there are a fair percentage that are having problems. Sign me up for Applecare at $244 at B&H.
I would expect that most refurbs are just 14-day returns, without any hardware defect, and maybe returns of older models from shops that overstocked on them. Some of them might have been defective units where the customer received a replacement.
Apple does not normally accept returns outside the 14-day return period. They might however choose to replace a unit instead of repairing it.