As I understand it: (not that I've looked very hard I admit).
P45 replaces P35 - Mainstream chipset
X48 replaces X38 - High-end Chipset
The 4 series is the new stuff and is as of this moment the current stuff that has 'just' come in.
I think there is the P or G43 that replaces the P or G 33 chipset? (was a budget end chipset and really not sure on the chipset numbers here).
EDIT: Just to confuse things, the X38 is essentially the same as the X48 but the X48 is a higher grade (better manufactured or cherry picked) component chip. As far as I understand it, the X38 has exactly the same features as any of the X48 boards, but overclocks may be better with the X48 (slightly) and note the 'may'. Also, the X48 "officially" support the 1600mhz fsb, which I don't think the X38 did. Because the X48 chips are better they run cooler. The X38 is noted to run hotter, perhaps too hot really.
Matthew