I like the Tom Zarek as a cylon theory. There are so many possiblities. I'm having trouble believing that anyone else (other than our famous four), on Galactica during the last jump/power outage, at the end of season 3 is a cylon. Reason being, is that they would have been sucked along with with the four cylons together. Again, I have some theories why and why not. One of my more fav reasons why is that the famous four (or five) are different is that they are half cylon / half human already (human connection to earth using "All along the Watchtower"). Full cylons know they can create half human / half cylon because they already have, but it has to happen in an uncontrolled setting (thus I'm not expecting multiple copies of the final five). They have tried (as we already know) to do so in an controlled setting (using science) to no success.
But if they fifth cylon is on Galactica....??? Well, during the last jump/power outage, easily there was some kind of trigger in our famous four, and as they get closer to earth their cylon brain slowly wakes up (finally waking up in the Nebula). With the fifth cylon on Galactica during the end of the season 3, it would have to have different wiring which, I'm guessing, will not activate until finally reaching earth (hence moving Galactica the wrong way so early in the 4th season).
So what does all this mean... a lot of theory and no real answers. What I am trying to do is pick out who is 'not' a cylon, not who is. Tom Zarek is still alive (I don't think we'll see any undead characters become cylons out of nowhere), he wasn't on Galactica at the end of season 3, so he is sitting pretty prime as a fifth cylon canidadate.