XP will be gone. But MS will allow a Win7Pro owner to change to XP-Pro. But they are not going to allow OEMs to sell PCs with just XP, other than netbooks... but Win7 is supposed to change that. Considering that the Win7SE is vastly limited, it should be a cheaper OS.
Vista will be gone too. What end-user would want to bother with Vista when Win7 is around? Remember, when vista came out - Retail/upgrade boxes of XP quickly disappeared. You can only buy XP-OEM versions now, until they run out of stock. Vista's market share will peak around Oct 2009 and go downhill from there.