Robert - the backordered message applies to the Ticket to Ride Card Set, which is right below the USA 1910 Expansion. Look closely at the web page and you'll see what I mean . . .
There's two backordered messages, one for the card set and one for 1910. One is only until 12/12/2006 (which is today on this side of the Pacific) so that's not a big deal... but then, backordered until Friday isn't exactly a big deal either.