With the clues provided and carefully examaning the cast/crew, I have come to a conclusion of who I think will be the 5th and finnal cylon.
Ok now for the reasons I think its Cottle (As per the clues)
1.He has been in the show since season 1
2.He was not in the TV film Razor
3.Helo and Athena are suposed to be "reacting and looking" at the spot where the 5th cylon is to be placed(between Tigh and apollo in the photo from scifi.com)
4.To go with #3 Dr.Cottle was involved in the whole contraversy with helo and athena's baby(hence why they are looking at him)
and 5. he would be a frakin awesome cylon
why I dont think its anyone else is they either dont fit the clues, or contradict the fact that the 5th cylons cant ressurect