Is the title 'The Walking Dead' referring to the main characters of the series?

Actually, the answer to this is probably found in the words of Rick Grimes himself, in 2015, the episode named Them.

[there's a long pause as the storm thunders over them]

Rick Grimes: When I was a kid... I asked my grandpa once if he ever killed any Germans in the war. He wouldn't answer. He said that was grown-up stuff. So... so I asked if the Germans ever tried to kill him. But he got real quiet. He said he was dead the minute he stepped in to enemy territory. Every day he woke up and told himself, rest in peace. Now get up and go to war. And then after a few years of pretending he was dead... he made it out alive. That's the trick of it, I think. We do what we need to do and then, we get to live. But no matter what we find in DC, I know we'll be okay. Because this is how we survive. We tell ourselves... that we are the walking dead.

Daryl Dixon: We ain't them.

Rick Grimes: We're not them. Hey. We're not.

Daryl Dixon: We ain't them.

(italics are mine)

See also this interview with Andrew Lincoln.

In addition, the topic is clearly referenced in Issue 24 of the graphic novel upon which the series is based, as seen in this image.

There is also "the fact that the title clearly refers to both the zombies and the survivors. The zombies are literally dead bodies that walk around (and eat people); the survivors are living on borrowed time, and are therefore, as the saying goes, "dead men [and women] walking"."

[Wad Cheber graciously contributed his time, comments, images and links to improve this post.]

Yes. But it's also referring to the zombies.

It's an example of dramatic irony of the most classic type that the title refers in a very obvious way to the fact that there is a bunch of dead people literally walking about the place, but in another sense applies to the people who are not (yet) in that state.

By issue 24 of the comic the writers got tired of being subtle about it and pointed it out. Unfortunately this led to people not getting the irony in the opposite way; one can often read that the "real" referent of the title is the characters, because the over-emphasis on "what is the canon answer about what happened here" is being applied not just to plot points, but to choices of words, as if there is some law that writers are allowed to only mean one thing at a time. (I'm reminded of the story of Joyce being asked by a critic why Ulysses had so many trivial puns, to which he replied in mock shock "what about the quadrivial puns?")

The comics address this very directly, at the end of issue 24 (no spoilers for TV, as I think this is still the prison saga).

At the end of this particular issue, Rick very plainly states "WE are the walking dead", after a brief, hopeless monologue discussing their current situation. He's clearly making a comparison between the survivors and the zombies, as he feels the survivors are similarly doomed and also endlessly seeking something out of their control (in their case, mere survival).

The title has a double meaning, as the writers and Kirkman have hinted at, and flat out said, numerous times.

On the surface, it's about the "walkers" aka "zombies" "biters" "geeks" etc.

As far as the theme of the series, both show and comic, it's a bit deeper. The title references they (Rick & crew) are the ones who survive amidst all the death around them, knowing that any of them at any time could be next, but they keep going on.

Just like the user above showed, there's two thematic tentpoles in the comics (only one in the show so far) where Rick says "WE are the walking dead" where he explains just that. Then later, he says "We are NOT the walking dead" as the series comes to its conclusion.

It's part of the reason why I liked the show and the comics, after avoiding them for years because I thought it was "just a zombie show". It's more than that, which is why it's a standout piece of modern fiction.

The phrase "walking dead" is also used in S3 E15 of Supernatural, and refers specifically to zombies. The episode aired May 8, 2008-long BEFORE TWD began on October 31 of 2010. Perhaps TWD writers were fans, and decided to use it?