Depends on where you buy them, what brand you buy, and what kind of licenses
you buy.
I would expect you to be able to get XP Pro if:
1) You buy (or have purchased) Volume licenses. These can be "downgraded"
from Vista to XP if you so choose. Ask your Volume license reseller.
2) You purchase the "business" class of hardware from various vendors (Dell
Optiplex Desktop or Latitude Laptop series for instance). They usually
continue to "grandfather" older OS versions for their business machine
customers for some time. But not for their consumer customers (Dell
Dimension or Inspiron, for instance).
3) You do it yourself after delivery (assuming you have Retail full versions
of XP Pro available for re-install).
-Frank