I think there are two ways to look at this. First of all, the fact that Locke is basically appearing in two places at once is not all that disconcerting given the propensity for the island to move forward and backward in time. For reference, see Emma Watson as Hermione Granger in "Harry Potter and the Prisoner of Azkaban" when she uses a "time-turner" to alter her appearance in time and, thus, be in two places at once. So, I don't believe in this instance that Locke's body is necessarily proof of his death; merely a glitch in the time-traveling conundrum of the island. Perhaps, his body and his appearance in the statue are evidence of his slipping through a wrinkle in the time-space continuum.

On the other hand, if we take Locke's body to truly be dead, then we're faced with the problem of who, or what, exactly is inside the statue? For this scenario, we assume that "dead Locke" is merely a proxy for Christian, Jack's father, so that Jack and the other members of the Oceanic Six can get back to the island. This also enables Jacob's rival (who is unnamed at this point) to take the form of Locke to assassinate Jacob, via Ben Linus, through a loophole in the karmic system of the island.

Given all this information, I'd have to say that John Locke is dead. The "John Locke" inside the statue is not Locke, rather a loophole for Jacob's rival to gain access to Jacob.

Oh, and, um, spoiler alert!