The 8.9-inch Intel Atom-powered Acer Aspire One is in stock, if you happen to be in Canada, from NCIX.com (they have an American side of their store, but no idea if they ship here, and the models could be different).
They are selling the one they have in stock for $379.99 CAD (US = around $377). A real-time check of their stock shows that in some warehouses they have 10+ available.
Note that this is the three-cell battery version of the Aspire One.
– 512MB Memory
– 8GB Solid State Drive (SSD)
No idea on when the Windows XP versions will be available.