Desmond's flashback was different in that it was a time travel, dream-like experience. It was a combo of the real island life and the past life with penny. It was cool, but very twilight zone. Although creative, it was another bizarre tangent for the lost writers, who now want viewers to believe that Desmond travelled back in time and can now also see the future.

I honestly don't think time travel is possible in real life, and even in a electro-magnetic blast one could not travel back in time. Its also sad to realize that Desmond is probably doomed and will never see penny again, since the most important thing he'll ever do is push a button. So raising kids is out of the question for him.