Desmond is Placeholding for Jacob and He Cured Sayid by Cos

Just putting this out there...

I believe Desmond is the force on the Island right now that stands in direct opposition to Smokey.

Smokey = Darkness
Desmond = Light (for now)

We've seen Desmond get blasted with the extremely bright white light in the makeshift cabin, as well as when he turned the failsafe key. And we were told his body was capable of absorbing EM (probably from his many years of pushing the button, gradually building a tolerance to EM).

We were also told that you can't let Smokey talk to you, or else you're his. Well, if Desmond is the Light-side equivalent of Smokey right now, I would say the same for him. And what did Sayid do?

He let Desmond talk to him.

The key is what we saw as a result of Richard getting ping-ponged between MIB and Jacob in Ab Aeterno. Richard let MIB talk to him when he first got to the Island, and as a result did what MIB said. When he went to get all stabby with Jacob, he was turned around when he let Jacob talk to him.

Smokey can talk you into the DarkSide, Jacob (and for now Desmond) can talk you out of it, at which point the best Smokey can do is hope you'll use your free will and come around on your own. As he said to Richard, "If you ever change your mind, you know where I'll be."

I believe that Sayid is back on track, and now conning Smokey. He'll probably make some kind of sacrifice, and throw off Smokey's plan at some crucial moment.

I don't necessarily believe Desmond is a candidate, or the one who will replace Jacob for good. I took Widmore at his word when he said Des would have to make a sacrifice. I think he's just holding the place for the one who will ultimately take the job for good (Probably Jack, but I'm pulling for Hurley).