The following entry containers spoilers - although not of the normal sense. These are secrets that have been uncovered on some of the various fake web sites (well, real sites built for fake companies) that surround the show. First off is this letter:

This gives us a place, Zanzibar, that could be the island for the show, but that can't be right since Zanzibar is somewhat bigger than the island in the show... and populated. Of course - Dharma/Hanso could have a facility there that just integrates with an island more away from Africa.

Either way - it gives us an area to consider. It also tells us that Hanso is in deep with the government, which makes sense. You can't just setup shop on an island without someone knowing.

Now take a look at the next document. It is the most interesting:

Life extension. Wow. We know that Dharma set up shop on the island due to the "unique magnetic properties." We also know that something went wrong. Perhaps they were experimenting on humans? My theory is that the Others are the result of this experiment. I think they went.... not insane... but... something else. I also think they became powerful psychics. This is where we get the manifestations (polar bear, Jacks father, etc) - their psychic powers simply reflect off the passengers.

I also think that the whole reason for the number entry task is to ensure you haven't "gone over" to the Others.

Next we have another letter from GHO. This is available online, but I did a screen capture and sent it here. I'm not sure if you can have a copyright for a fake corporation, so until someone complains, I'll leave it up:

This obviously smells like a bribe of some sort. My assumption is that Hanso simply paid GHO to look the other way.