Well, if shorting pins 6 and 8 with the POWER SW lead or by touching the 6/8 pins with a screwdriver do not power the system on, a short to the MB is likely causing the system to not power on. The MB must be mounted in the case properly (use the brass risers to keep the MB from touching the case metal). Also, be sure to check that every cable is attached firmly (Sata, IDE, etc.) as well as the RAM, video card and PCI cards are firmly seated. Perhaps reseat them. These could also cause the system to fail to power on if not installed properly.
The green light likely shows the MB has power, but a short would still prevent the system from powering on.