Blurb: Desmond wakes up and realizes he is back on the island.
Ryan
Alan Sepinwall
EW pre-episode blather
EW Recap

So sideways-Desmond can “see” what happened at the island and with Penny when he’s undergoing an MRI. MRI, of course, is the initialization for Magnetic Resonance Imaging. Desmond (on-island) has been through a major electro-magnetic storm and survived when he flipped the fail-safe key in The Swan station; apparently Widmore knows that, since he (Widmore) wants to test Desmond again to be sure he can withstand another event. The implication seems to be that there’s going to have to be yet another cataclysmic event like that to resolve things.
Sometimes I think the final two-hour episode of “Lost” is going to consist of all the Losties sitting around drinks at a circular table in a bar somewhere in LA. They’ll be explaining their behavior throughout all six seasons to one another. That’s the only way we fans are going to get all the answers.