What if the last cylon is Billy? I mean it seems like the ones we know descend in age, or it seems that way. Billy was quite young, and theres really no reason to pop a new character up on everyone. Deanna did react strangely when in the temple when she "saw" the final 5, like she knew them all or had at least encountered them all.
I'm going with the long shot and gonna say it's Billy.
