Everything sounds like a normal motherboard LED except the part "when the main power button is pushed, this LED comes on". So this light NEVER comes on when the power is connected until you press the power button? So for example, your computer is off, power is off, you plug the power into your psu, turn the psu on, this LED doesn't turn on? And then you press the power button and THEN this light turns on?
If that's the case, I think there may be a problem with your motherboard. If you can get a friend's motherboard or something then you can test.
ASUS A7N8X-E Deluxe
AMD Athlon XP 3200+
OCZ 1GB Platinum PC3200 Rev2 Dual Channel
ATI Radeon 9800 Pro
Enermax Noisetaker 485W
2 x 80GB SATA Maxtor Diamondmax 10 on RAID 0