Originally Martha and Jonathan Kent died of natural causes, after their adopted son Clark Kent graduated college, and never saw him as Superman. Except one time when Jonathan was able to travel, briefly, to the future.
Then, after Crisis Of Infinite Earths, they were rebooted, now younger when they found Clark and both lived to see their son grow into an adult.
Now, in the New 52 Action Comics, Grant Morrison has reverted the situation. The Kents were still not elderly when they found Clark, but they do die before his time, according to the io9 preview, due to injuries sustained in a car accident. We also get Jonathan Kent's speech from the deathbed, urging Clark to use his powers for the benefit of humanity, just as he used to have done.
But this is a time travel comic, with Superman fighting the same foe in three separate timelines simultaneously. Will there be room for a little time travel to the future again? Or from the past?