So, help me understand: When we had the flash sideways, was that Desmond actually floating back and fourth, or was that just us watching this alternate timeline?
In the episode 'The Constant', it is my belief that Desmond is traveling back and forth between the alternate realities, as opposed to having a flashback, like they implied (at this point in the series, flashbacks were the norm). When Desmond goes back and sees Penny, and she is hesitant to let him in, I believe this is the first appearance of the Flash Sideways timeline. I think him meeting Penny in the FST will end poorly, and he won't see her again until that scene happens in real time (he goes to her apartment, gets her phone number, etc.)
In 'The Constant', Desmond is getting the time sickness from traveling in between the Island and the freighter, and he is switching back and forth between realities. The time sickness kills almost everyone who gets it - their brains can't handle jumping back and forth (see - the driver from last night in the original timeline, Eloise the rat), but the rules don't apply to Desmond, so he safely switches between timelines.
As someone hinted at before, Desmond's famous line 'see you in another life brother', has actually been foreshadowing this whole time. He is special, he can consciously exist in both realities, and now that he knows this, he has 'something' to show the rest of the survivors (in our real life, the plane crashed, etc.)
It's interesting to note how he acted after Widmore did the experiment on him. He agrees to do whatever Widmore wants, which makes enough sense, but then he immediately goes with Sayid, without any hesitation. Has he lost his free will? Or does he just realize now that the Island has a purpose for him, and he's letting it play out.
I LOVED all the imagery in this episode - the McCutcheons (sp) whiskey, the panting in Widmore's office, the Desmond/Charlie drowning scene, etc.
And, of course, it was GREAT to see Faraday again.
An interesting thought that hadn't crossed my mind until you guys brought it up - it does seem like the FST is like, the what-if timeline. A lot of the people get exactly what they've (probably) always wanted. Let's think of some examples -
Jack has a son, family
Locke has Helen and his father in his life
Kate is free
Claire keeps her baby
Charlie is a big time rockstar (not a one hit wonder)
Desmond has Widmore's approval
Faraday is a musician
Ben is close with his father and Alex
Dogen's son is not dead
Hurley is lucky
But they don't seem happy. It seems like a 'be careful what you wish for' type of thing.