First, if you need a spoiler alert for The Hobbit, stop whatever you are doing, take the day off work or school, send the kids to grandma's, and read The Hobbit now. Twice. Or 133 times, like I have.
That being said, I like to put my predictions on the table for all to see, so here goes.
Smaug will not die in the second Hobbit movie "The Desolation of Smaug". Instead, he will die in the beginning of the third installment, "There and Back Again", to be released in December of 2014. I have believed this ever since I heard the book was to be split into three, and despite conventional wisdom on Tolkien-related websites such as TORN. Why do I believe this?
The first reason is material balance. The first Hobbit movie, "An Unexpected Journey", covered six of the nineteen chapters of The Hobbit, plus background material that covered about two chapter's worth of material. Each chapter received about 20 minutes of screen time on average. For Smaug to die in DoD, eight chapters would have to be covered, finishing simultaneously with Not at Home (the dwarves capturing Erebor) and Fire and Water (Smaug razing Laketown, then being slain). There is also the Dol Guldur plotline following Gandalf and other miscellaneous side material which will again consume about two chapter's worth. The third movie, TaBA, however, would only have five chapters left to work with, two of which are short and post-climax. Likewise, for both stylistic and coherence reasons, the Dol Guldur plotline will end in the second movie, so it is hard to see where Jackson will pull a lot of secondary material from that fits within the story arc.
In contrast, if the movie ends where I think it will - at the beginning of Not at Home and Fire and Water, about 20-30 minutes of material can be shifted from the second movie to the third, creating an approximate balance between the two. Specifically, I think the movie will end with Smaug beginning to raze Laketown, highly contrasting scenes of the elated dwarves reclaiming their treasures, both interwoven with the climax of the Dol Guldur plotline. Not only does this ending provide balanced material for the two remaining movies, but it is also a much better ending to a mid-series movie. It is all Empire, no Ewok: some plotlines going well for the good guys, others disasters, others ambiguous. Rather than finishing off the two main antagonists, leaving one alive at the peak of his sound and fury would be something to draw people back to the theatre again next year.
A third reason I believe that Smaug will not die this year is the new teaser trailer for DoS, which contains precisely zero scenes from either Not at Home or Fire and Water, but plenty from all the rest. Enough said.
Smaug will live...until December 2014.