Elite Bastards posted a preview on the Intel X48 chipset

So, what do we have by way of improvements in X48 over its forebear? The simple answer is - Nothing major. Most of the under the hood changes to the X48 chipset concern tweaking the design and components with a mind to adding official support for Intel's latest, 1600MHz FSB CPUs. While X38-based motherboards will often be able to do this unofficially, X48 should make for better stability at these higher FSB and memory speeds, as well as allowing for better overclocking of such parts.

In short then, official 1600MHz FSB support and a better optimised memory controller (again with 1600MHz support in mind) to go with it is the order of the day here.

Intel X48 chipset preview - ASUS P5E3 Premium motherboard