I believe that the Bible makes it pretty clear, by the very fact that Jesus died, that his kingdom is Heaven, and earth is more or less Satan's domain. Since Christ did die and return to Heaven, his quote can easily be taken literally.
BTW... I'm a believer of science and evolution, and a god somewhere...
Anyway, he didn't say that people witnessing him speak would still be living when he returned to earth, but when he returned to his Kingdom, which was Heaven, and took place shortly thereafter.