Uncharted: Drake's Fortune demo has been confirmed for early November. What has remained a mystery, however, is what level we will be playing. Reading through this article I found an interesting bit of information on the cinematics of the demo, which could confirm what level we will be playing. The article states that the demo will feature a cinematic clip of Nathan and Ellen parachuting onto the island after their plane is hit by flak. This would suggest that the demo will be the first level on the island.
After hitting the jump button click on "Cinematics Animation" to read more.