I was listening to the Lost podcast that they did just after last week's episode, and they made an interesting comment saying that we're NOT seeing two alternative realities. Maybe earlier thoughts were correct, that the "side-flashes" come AFTER what happenes to the on-island losties at the end of the show.
I was also reading some other interesting theories on another Lost forum, and this one got the wheels churning. We've seen four ancient remants on the island of a previous civilization/society that was there before everyone else... and they match up pretty well (so far) to the Seven Wonders of the Ancient World:
Temple of Artemis - The Temple - Artemis (according to Wikipedia) was the "goddess of forests and hills, childbirth, virginity, fertility... she even assumed the ancient role of Eileithyia in aiding childbirth". Hmmm, child birth...
Colossus of Rhodes - The Statue - Even though the Colossus was a statue of the Sun God Helios, the coincidential nature of this and the next two are too much.
Hanging Gardens of Babylon - The Orchid - The Orchid looked very much like a botanical garden from the outside... and although the ancient aspect to this area was actually the Well with the time-shifting wheel at the bottom... it's still quite a strange coincidence.
Lighthouse of Alexandria - The Lighthouse - I would think this one is fairly self explanatory. (Also interesting, but unrelated, to note that Dharma tracked the island back in Los Angeles using a station called the Lamp Post.)
This leaves three structures yet to be seen on the island, such as the Mausoleum of Halicarnassus, The Statue of Zeus at Olympia (although there have been previous references to a volcano on the island), and the Great Pyramid of Giza (again, volcano?)