I was wondering this when we watched The Polar Express the other night. In the movie, there's a kid from a poor family who says that Christmas "just doesn't work out" for him, hinting at the fact that he hasn't received Christmas gifts in the past, and is surprised and excited when he receives a gift from Santa late in the movie. And I was sitting there thinking, "But if Santa is real in the movie, why wasn't that kid getting a gift all along? Why this year? Was the kid 'naughty' in all the previous years?" It didn't make sense.
That said, we do Santa in our house, so I'm interested to see an answer to this question from someone who does Santa. As much fun as I have doing all the Santa stuff, we don't do the whole naughty/nice part of it, and I certainly wouldn't want my kid thinking that all poor kids are "bad."