Actually, what I heard was that the
486sx was a
486dx chip with a bad
FPU and that, during testing the chip people said, hey, instead of trashing the chip, lets add some chip
kung-fu to the chip so that we can disable the fpu portion and sell it as a cheaper chip, thus lowering the apparant failure cost of the 486 models.
I've heard this as being the reason for lower speed pentiums as well though, so who knows if this is true.