I just purchased an off-the-shelf 92mm case fan, plugged into the 4-pin sysfan header, and noticed right away after powering-up that the air flow is reversed – from outside to inside the case. My old (loud) fan spins correctly, but it's only 3-pin and shouldn't matter as all that's missing is PWM.
Now before you think I mounted it wrong, the flow directional arrow points towards the outside of the case, the concave part of the blades is on the case side, and the fan bracing is facing outward as well as the label on the rotor.
Now the weird part. Looking from inside the case, the blades should be spinning counter-clockwise in the direction of the rotation arrow on the fan. Instead, not only do they spin clockwise, the "teeth" portion of the blades are leading toward the counter-clockwise side, but instead the blades look to be going backwards. I tried some suggestions of forcing the spin in the opposite, but the fan doesn't want to go that way. Is this just a fan wired backwards at the factory or am I missing some fundamental and embarrassing point?