Feature-wise the X48 will be exactly the same as the X38 chipsets. The difference is that the X48 chips are cherry-picked chipsets which will overclock even higher than standard X38 chipsets but they will be very limited in supply and motherboards with the X48 will obviously carry a premium price tag.
Today FUD Zilla heard that ASUS will be one of the first to get their hands on the precious X48 chips. The new ASUS ROG X38 motherboard might feature the chipset and may become the best overclocking motherboard available on the market.