TC Magazine claims 1600MHz FSB support won't be limited to the Intel X48 chipset. The X38 will support it too as ASUS and Gigabyte are already claiming their boards will support it:
Although Intel has yet to announce its plans to introduce a desktop processor with a 1600 FSB, the two 'pluses' above are certainly interesting selling points for the X48. But, at least one 'new' thing about the X48 is already available with the X38 as, at least Gigabyte and Asus claim that their motherboards feature support for 1600FSB CPUs. As for the enhanced memory support the X48, some Asus boards are already promoted as working with DDR3 memory at 1600, 1800 MHz and more. That tells us that there won't be much to the X48 if it relies on the FSB and memory speed alone, especially as it is safe to assume that it will be more expensive than the X38. Wait and see.