It is quite common knowledge that
Vader and Anakin Skywalker (Luke's father) are the same person.
However, this is technically a spoiler (I think there are still some folk around that haven't watched the Star Wars films). It pops up in some questions, and sometimes even in titles. We had just a brief discussion in a question's comments, and I thought it should be decided for good.
Note: I'm not leaning either way, and neither is this meta question. The up/down votes should be used for relevance, not whether or not it should be considered a spoiler.