Note: This is an older version of my prediction "Hollyleaf is Alive". This version was written before "The Fourth Apprentice" was released.
I could be wrong, but it seems to me that most people here seem to think that Hollyleaf is dead. I disagree with this opinion, for several reasons:
First of all, it should be noted that Erin Hunter refuses to reveal any information on whether Hollyleaf is really dead. (See Erin Hunter Chat #6) This is one reason why I believe that Hollyleaf is not dead. After all, if she is, what's the big secret? She could eaisly be brought back into the series, it wouldn't be the first time. After all, look at Graystripe.
Another reason for my belief that Hollyleaf is not dead has to do with the cause of her 'death'. Think about it, of all the ways that Hollyleaf could die, why have her die in the tunnels where no one can confirm her death? I believe this was done intentionally. There are probably many more undiscovered entrances to the tunnels, leaving pleanty of potential for Hollyleaf to return at a later date.
Another thing I noticed is that most of the books avoid directly saying that Hollyleaf died. Instead they usually say: "Disappeared" or "Was Lost". This could be another indication that Hollyleaf is still alive.