It's always confused me too, originally I assumed that it shattering was symbolic of Arwen forgoing her immortality and dying and that it only broke metaphorically. But then on one of the EE cast commentaries Billy or Dom suggest that it was meant to break, which is why Aragorn doesn't have it visibly on him as he rides from Gondor and at the Black gates.
I guess, because the film makers pretty much invented the Evenstar (though it does look a bit like the jewel Arwen gave Frodo in the books) it was on the whole more of a symbolic device than as a plot point, so it didn't matter if it was ambiguous.
In terms of Arwen's vision I always interpreted it that Eldarion was wearing the Evenstar as it was a really simple way to show the audience that this seemingly random kid was Arwen's son and that this would be the outcome if she chose to stay in ME- rather than it being the literal future- which I guess, to me, means it doesn't matter whether it was meant to break or not
Though if it did smash, its a shame because it's such a pretty thing to drop on the floor!
the whole thing is odd, it seems real during the extended edition and doesn't seem to be on when they ride to the Black Gate, could be hidden, could be a mistake, who knows.