I think the show has now proven that
time is not linear - that is, within the confines of the show itself.
What I found amazing was the idea that time-travel allows you to exist in two places at once - or should I say, two perceptions of time at once, since technically the conscious of Desmond (and Minkowski) was alternating between point/s in the past and as well as the present (or future-present, if you will).
We saw that neither Des nor Minkowski were able to control their time-hopping experiences (I'll refrain from calling them 'abilities' for now since a distanct lack of control was apparent in both of them) however with the aid of Faraday, Des was able to learn enough to understand that a "constant" would
anchor him in one place in time (i.e the present). What I found amazing was the idea that they were able to flick in and out of their pasts, and alter their present through a 'combination' of going about their past differently (i.e. Des contacting Penny and giving her vital info as to the call he was going to make in the future-present) and by establishing contact with their constant in the future-present. Of course Minkowksi was unable to do this, but Desmond's actions are simply jaw-dropping for any time-theorist/enthusiast.
Why? Because not only did he do the above but if we look at this on a deeper level, we can see that he 'replanted' (or rather implanted) new memory into his current self by taking future knowledge and understanding into his past consciousness.
Yes, "you can't change the future" (according to Faraday), but you
can alter the past in order to 'create' a 'different' future via a 'new' past. As seen when Des was walking away from Penny's house with the sensation of an optimistic future washing over him (i.e. the knowledge that he and Pen would speak on Christmas Eve 2004).
Which begs the question - did Des know he would go to the island? No - because like Faraday he "forgot" - the "new memory" is happening in
'real-time' and over-wrote the old memory pathway. The only time when Des was consciously living in 2 places (or times) at once was the moment he gained his "constant", and even then it was more a sensation (new memory), than anything.
Incidentally, I'm now even more excited by the prospect of Ben being able to do a similar thing - only on a more controlled level - after all, it would explain how he is always two-steps ahead (although the fact that he has fake passports and currency, slightly negates this idea, perhaps, as it gives his 'movements' a more
tangible reality)
Not saying i'm 100% right, but I wanted to get my current thoughts recorded. Feel free to add and update your own interpretations if you want.