Ok, here's my off-the-wall theory before the episode premieres:
We need the Oceanic Six off the island under stressful or unfortunate circumstances, right? We need the freighter to go boom (otherwise they'd all be rescued). We need some people to be trapped on the island. Now, if I was writing the show, here's how I'd do it:

I would have everyone tie up whatever they're doing at the moment first. That's important. I have a feeling that the Others and the mercenaries are going to have a little turf war, while Jack and Sawyer rescue Hurley in the background, with a little help from Sayid and Kate (who show up with the Others). That puts almost all of the Oceanic Six together, minus Sun and Aaron (who are together now, remember?).

I have a feeling that Desmond and Michael are going to work frantically to defuse the bomb, but will realize that they need Sayid, so they send the boat back to get him with Sun aboard (in case the bomb goes off). Keamy gets killed, boat blows up (remember the device on his arm?), and the boat carrying Sun, Aaron, Kate, Jack, Hurley, and Sayid returns to charred remains. They turn around to head back to the island, but when they get close enough to see the island, it suddenly vanishes (Locke succeeds in moving the island).

With no boat, and no island, they pick a direction and head towards it, eventually being rescued. It's just an off-the-wall theory, but it would explain why they're conflicted about going back, why only 6 were rescued, and why no one can find the island again.

And, as an added bonus (credit goes to my little brother Kyle for realizing this one), think about the day that these things are happening...around December 26th, 2004... that's right, the 2004 Tsunami, which eminated from a point in the Indian Ocean near Thailand... where the island could possibly be located. If the island was to suddenly teleport itself to a completely different location, it would create a total vacuum where it was. The surrounding water would rush in suddenly, maybe an earthquake... bingo bango bongo: you've got a Tsunami! Is it possible that Locke and Ben create the tsunami by moving the island? I guess we'll have to see...

Alternately, you could mix and match any of the above variables: maybe they defuse the bomb and head towards the island, only to "move" with it when Locke succeeds, leaving the six stranded in the boat elsewhere... maybe Sun and Aaron meet up with the rest of the 6 and get "moved" before they can get to the boat or even off the island... maybe the boat blows up, but Frank gets the 6 off in the helicopter, which crashes after the island "moving" emits a huge EMP burst...

I can't wait for tonight's episode!