Recently the SciFi Channel has been airing the old "Land Of The Lost" TV series as prep for the new movie of the same name, and I found in it many time travel elements in common with Lost. Although there isn't much depth to the average episode of the LOTL series, many of the stories were written by established and respected Science Fiction authors, such as David Gerrold, Larry Niven, and Ben Bova. One of the eps that I found particularly interesting was the one written by D.C. Fontana ("Elsewhen"), who is well known for her TV scripts and stories, notably the Star Trek series. BTW, "Elsewhen" is also the title of a novella by Robert Heinlein about parallel universes.
I looked up the unusual title of the episode, and found that it pertains to time travel theory, particularly concerning light cones, which we have seen notations on in screencaps of Daniel's notebook. Doing a search on "elsewhen" came up with the first image at the top of this post, which is very similar to figural notations that I've posted previously of Stephen Hawking's (http://perditascientia.blogspot.com/2008/07/visual-aids-revisited-fwiw.html). The website where it resides (http://www.elsewhen.tv/) shows nothing other than the image. But I found some text on "elsewhen" at another site (http://www.wisegeek.com/what-is-the-elsewhen.htm) that covers some interesting points: "Let us think about time in a linear fashion, with the future stretching out before us, and the past behind us. Those areas of the universe that we theoretically could have had an influence on are said to be within our past lightcone. Those areas that we can have an influence on (if we could travel at the speed of light [the"universal speed limit"]) are said to be in our future lightcone. Anything in the past or future lightcones are said to be in our "locality"; everything else is in the elsewhen [or in parallel]."
This thought, and Lost's implications of Kerr's theory on concentric shells of time, help explain the possibility of parallel existence in separate times, among other things. In Fontana's LOTL story, Holly travels down a mysterious hole in a cave to get her brother and dad out of a jam with the Sleestaks. While dangling on the rope in the hole, Holly passes through an upside-down (parallel?) world.
She also meets a woman who walks out of the mist of an inter-dimensional portal who gives her advice about the present and future. Holly notices that the woman has a large scar on her arm. Later after cutting her arm while saving her dad and brother, Holly sees the woman again and realizes that it's really her own grown up self that she is talking to. On Lost in Season 5, we find out that people like Sun could be existing in 2007 in two separate places at the same time, i.e., on the island after the flight 316 "landing", and back in the world as one of the Oceanic 6. As well as the LBs existing in two places in 1977 (if they were born by then, haha). But so far we haven't actually seen a future Lostie speak to their past self yet, other than Daniel leaving notes for himself in the past and future, or Locke sending himself messages across time via Richard. And, that brief moment when we thought that Locke was existing in two timelines, before we found out that one of them was really Jacob's nemesis in disguise.
As an added note, there is another episode of LOTL, "The Circle" (written by Niven and Gerrold), that is also very similar in concept to Lost. In it, our three heroes meet up again with their Sleestak "pal" Enik, who once again is trying to get back through the portal to his own time, while not caring about the Marshall family's troubles. Enik, Rick, and Will have a lengthy discussion about how nothing can go through a temporal doorway properly without something of equal energy passing through the other way at the same time. A cosmic swap, if you will. But Enik surmises that the Marshalls entered without the swap, so they are actually dead in their own time, but also existing in the Land of the Lost, and that they must leave in order to fix the paradox. The Marshalls assume that they can get home if they complete the paradox, so they go through the portal which only brings them back around to their initial entrance into the Land of the Lost. The catch is, doing this fixes the paradox but allows only Enik to go back home, and leaves the Marshall family to re-arrive in the past, and live their time there all over again. Sound familiar? So the moral of the story is, with liar friends like Enik and Ben Linus, you don't need enemies.