Until recently I had absolutely no question in my mind as to why the TV series 'The Walking Dead' was titled exactly that - because of the zombies, you know, corpses that have come back to life and all.
However recently I've started to doubt this. The 'walking dead' when used in a general sense is used to refer to someone who it actually alive... but doomed.
For example saying "he's a walking dead man" would mean that the person is currently alive, but has signed his death wish in some way.
Similarly the phrase "dead man walking" on Wiktionary has the meaning
A condemned prisoner walking to a death chamber or other place of execution. Someone who is soon to die. Someone who is about to face an unavoidable loss (though the person may not realize it)
So could is be that 'The Walking Dead' is in fact referring to the main characters, and reflecting the dire situation that they are in? - Rather than simply refering to the zombies.