This has been thrown back and forth for the last couple weeks since the DVD came out. The new exchange with the Emperor has how it is possible that the one who destroyed the Death Star could be the son of Anakin Skywlker. Earlier inthe film Vader says "...Skywalker is with them!" and proceeds to the Hoth system.
Some people have been arguing about the continuity error there. We had a brief chat about this over the weekend. We basically distilled it down to this: Vader knew that the young rebel's name was Luke Skywalker but didn't realise this Skywalker was his son.
I can't imagine that, in such a large galaxy, the only Skywalkers would have been Shmi, Anakin and Luke.