Well, the Fifth has to be someone on the Basestar at the end of what we've seen so far. Because Deanna said the fifth wasn't in the fleet, only four of them were. If the 5th wasn't in the fleet it had to be someone Deanna knew to be on the basestar.
And the only major characters on the basestar were Roslin, Baltar, and Helo. There were some recurring minor characters like Hotdog (who is Eddy's son IRL btw) and Seelix on the basestar, but it would be kind of lame to make the 5th one of them.
I don't think it could be someone who is dead, because I don't think Deanna had time to be briefed on everything that happened since she was boxed.
The big question is... why wasn't the 5th activated with the other four at the nebula and again in the last episode we saw so far? The 5th has to be fundamentally different from the other four just like the final five are fundamentally different from the other cylons.