I'm breaking this article up into at least three parts. Part II will address hypothetical given meanings, and Part III will concern subjective, individual purposes. Please keep that in mind when commenting, i.e. don't complain that I didn't address individual purposes here since they'll be in Part III.
Part I: Objective Meaning
What is the meaning of life?
Short answer: there is none.
In order to properly address this question, we have to understand what we mean by "meaning" and "life." By "life" I mean the existence of sentient beings like humans in general, and my own existence in particular. When we try to attach a meaning to life, we really mean, why do we exist? But this isn't a "why" as in, "what are the events that caused our existence?" We are typically more interested in a purpose behind our existence. But for there to be a purpose, there must be intent, and for there to be intent, there must be a sentient being to have that intent. I don't believe a sentient being purposely created the universe or life or humans. Therefore, I don't believe there is a purpose.
My definitions of "meaning" and "life" in this article are non-negotiable. If you want to discuss other meanings of the meaning of life, post it on your own blog.
My answer - that there is no meaning - relies on sentient beings not having a sentient creator. Is that a reasonable assumption? I would say yes. I don't believe in unicorns because there is no evidence for their existence. Same goes for gods. I may not be able to prove that gods and unicorns don't exist, but that doesn't mean I should believe in them or even be completely agnostic toward them. I have already discussed the existence of gods here. If anyone wishes to further any of those discussions, email me and I'll open a new thread for it. (Blogger comment notifications don't work for me, so if you comment on an old post, I will probably never see it.) I'd prefer to limit this one to the title question.