Obviously the place was rigged to blow, not as a booby trap for intruders.... but as a self destruct to keep the hatch out of the wrong hands. I never assumed that Dharma rigged it to blow, rather I assumed it was the hostiles. The reason I thought that was because Mikhail seemed to encourage Locke to continue playing the game. It was no surprise to him where the game was leading. Originally, the 77 may have been an S.O.S set up by Dharma...but it seems to me that since communications are down it would now be useless. However, if the hostiles rig it to blow when someone tries to call for help...that makes more sense to me. Why Locke did not discuss the prompts with his fellow losties is a mystery to me. I would not have been able to wait to show them what was going on with the computer. On the other hand, why didn't Kate and Sawyer mention the place was rigged to blow? I agree that the losties need to start trusting each other enough to communicate these kinds of details! It would save a lot of trouble.