the fifth Cylon

It's got to be Ellen Tigh - Saul's (the XO of Gallactica) wife.

In the first season, she was mysteriously 'discovered' alive on a ship in the colonial fleet as a 'Jane Doe' and then brought aboard Gallactica and reunited with Saul. Everyone thought she was a Cylon then.. of course, her being dead again makes it difficult. We've never seen any clones of the the 4 new Cylons that we know of, but that doesn't mean that they don't exist.

It does seem like she's been dead for a long time to be just now showing up, but there definitely WAS a Resurrection Ship in range when Saul killed her for being a Cylon double-agent, even if she was doing it to protect Saul at the expense of the Revolution on New Caprica.

A good case can also be (and has often been) made for Kara Thrace, what with her disappearing for months and not remembering any of it, but since the non-lobotomized Raiders seem to be able to recognize at least Anders and probably the other 5, and they don't seem to be too squeamish about trying to kill Kara, I think that's probably unlikely.

My money is on Ellen. Literally.