I have no idea when it comes out. If it follows the same time frame as MGS2, it will probably be out Q4 2004 in Japan and the US, and probably Q1 2005 in Europe.
Having watched the trailer a few times, I am left wondering how going into the jungle will continue the plot from MGS2. I now believe the trailer is'nt for MGS3 but for the remake of Metal Gear (where you did shoot it out in jungles) for GameCube (MGS: Twin Snakes), which is also being shown off at E3. I guess we will find out soon enough....