They're not running Vista because MS is not ready to blow its marketing budget on funding a $200 laptop with the $800 additional parts that are required.
And MS is not ready to do that because Steve will go through all the chairs in the the reserve if he finds out that one of those laptops has been wiped to install (gasp) Ubuntu.

Just how long is MS going to support XP if new laptops are available with it on?

Are these poor people being sold the old XP that will not be supported soon, or are we being sold an expensive OS while the cheaper older XP will be supported for years and years? At least one of us is being conned, it looks like all of us to me.