One explaination I've heard is to do with your eye/brain frame rate. After certain speeds you start seeing the wheel going backwards because your eyes aren't updating fast enough.

It's the same reason why you aren't loking at your computer screen scrolling and updating. we can't process the information fast enough.

Let's say you have a four spoked wheel. To begin with the spokes are in the shape of a "+".
For an average car say a wheel is 0.6m diameter.
so it's circumference is: 0.6*pi = 1.9m
if the car is travelling at say 32m/s therefore the wheel is rotating at: 30/1.9 = 17rev/s

that value would have to be faster than the rate at which our brains can process the data for this phenomenon to occur.

Now, the spokes have obviously completed some part or more of one revolution of the wheel. If your brain updates the information from the eye at a speed at which the spokes have seemingly travelled more than 45deg (i'm guessing - probably further) but less than 90deg from their original position, then it will look like the spokes have travelled backwards.

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