Here was a spoiler from spoilerfix.com, concerning Desmond, in the first episode. It was posted on Aug. 31st, and now it suddenly makes a lot more sense.
Dont worry, it doesnt say how he got there or why he's there. Just his background.
Click to see Spoiler:08/31 - The season opener, "Man of Science, Man of Faith," a Jack-centered story, will introduce Desmond (to be played by Henry Ian Cusick), described as an avid daredevil who gets his kicks climbing the world's biggest mountains alone. Equal parts charismatic and gregarious and austere and zealous, it is unclear how Desmond will play into this season, but expect to see more of him as the story progresses.
Edit: Something got me to thinking. In season 1, every show took place basically in the span of a day. With tonight's ep, the timespan was what, 2, 3 hours?