People have suggested the numbers & computer and 108 as some sort of failsafe. I agree, kinda.
Hurley's freind and his buddy, and the research team, heard the numbers as part of a repeating signal. Desmond also seems very supprised that there's not been a major catastrophy outside ("The world's still out there?")
So I think needing to type the numbers in are a check-in. The illness, whatever it is, hits hard and fast. From first symptoms to death is 108 minutes. So when the alarm goes off, Desmond has to send out a signal saying he's still alive, by typing in the code, that we saw on the outside of the hatch. This implies that the numbers are some sort of identifier.
Only one problem... the signal isn't going out as the numbers anymore. Madame Insanity changed it to her message 16 years ago. So it couldn't be an automatic failsafe, or it would have gone off.
What if this Dharma group either believed, (or was about to instigate??), a plague of some sort was coming. The seeded locations all over the world with humans, complete with supplies and antidotes, with the intent of rebuilding humanity after the plague ran its course. Monster, shark, magnets, all could be designed to keep people away from the island, to fend off the plague from the haven that Dharma made (Which makes you wonder where some of the other havens could be...)
Perhaps Adam and Eve were the first volunteers for this isolation. They lived the rest of their lifespan in the hatch, and were 'buried' on the surface (The black and white stones? There's a GO board in the hatch.) Someone replaced A&E, something happened to the replacement; possibly involving the French team, since, arguably, Rousseau has been on the island longer than Desmond (16 confirmed years, vs estimated less than 12 from the Jack-Desmond meeting), and Desmond replaced them.
Could the 'him' Desmond is waiting for be his replacement?