I don't have any information to back me up, so my thoughts are just that, my thoughts. Let me just put those out here and then you can decide whether or not they have merit.
Heat, like most things that flow, take the path of least resistance.
Temperature likes to be uniform. If you put two objects of different temperatures together and leave them that way, over time, the temperature of the objects will move toward each other's temperature until they match (assuming no other outside influences).
Heat rises in air. As the air above an object heats up from that object a small convection current will flow in the heated air moving the heat up and away from the object.
It's really a lot more complicated than this, but this is enough to get the point across. So, there is no simple answer.
Scenario one: The power supply doesn't have enough air flow to move all the heat it generates away from the power supply so some of it moves up into the electronics sitting on top of it. The electronics generate their own heat and add to the problem. If you have the electronics fan blowing out then you draw more of the excess heat from the power supply over the electronics causing the air flow across the electronics to heat up more than if you had the electronics fan blowing in to counteract this additional heat. This assumes the fan is moving enough air to overcome the rising heat and remove the heat generated by the electronics.
Scenario Two: The power supply fan is sufficient to move all the power supply heat out the end of the power supply away from the end of the power supply where the electronics are mounted. In this case the fan blowing out would aid the natural flow of heat giving you the best cooling situation. In this case the fan blowing in would be bucking the natural flow and could cause a reduction in cooling.
So, until I get my X-carve (it was supposed to ship today, but got caught up in the parts problem) I can't test any of this.
My initial thought is that I will most likely start with the fan blowing out until such time as I can test it to see if the direction makes any difference at all.
Basically Rusty has the answer, you need sufficient venting and enough air flow and you can overcome most heating issues.
If the fan moves enough air it could be a moot point.