Some people have said that the original timeline is still there, but Nero and Spock have vanished from it by going back in time, and by changing the past they've created an offshoot timeline.
That makes no sense.
In the Trek universe, if you change the past, you've changed the past. (Haven't ANY of these people seen "City on the Edge of Forever"?!?!?)
Someone did suggest something, tho'...
What if the black hole that swallowed Nero and Spock didn't just send them back in time, but sent them into an alternate reality as well? (Something similar happened to a Constitution Class Starship in a two-parter on ENTERPRISE.)
This makes a little more sense, seeing as the tech on the Kelvin looks a little too good, and Kirk's mother was using a TOS style communicator, which supposedly wouldln't be used by Starfleet for another 20 years or so. (The ones in THE CAGE were VERY different from the TOS versions.)
I'm pretty much settled that this isn't an offshoot of the original (which wouldn't be possible, considering previous Trek stories), but is just a parallel universe...a whole new reality for us to play in.
As for Nero and Spock...
Maybe they're actually from this reality too, and they DID just go back in time. If that's the case, then "our" Spock is still safe back in the original Trek universe, working on Unification and living out his remaining years.
I hate to think of "our" Spock being trapped in the past of an altenate reality for the rest of his life, but on the other hand, it's a neat idea that he's made the transition with us and is still there as a connection to the original Trek universe.