According to the government paper printed in Federal Register dated 19.
April 2006, Mercedes-Benz does list S450 for the US market in 2007.
Scroll almost all the way down to see the list of vehicles.

http://a257.g.akamaitech.net/7/257/2422/01jan20061800/edocket.access.gpo.gov/200
6/06-3692.htm

However, the date is speculative. My money's on October launch as it is
often the traditional US launch date. The website is notoriously behind
when it comes to new model launch. For instance, the E55 Station Wagon
wasn't even listed in the website until six months later. I became aware
of E55 Station Wagon in the USA when I saw the listing for the slot to
the first or second E55 Station Wagon in eBay in December 2004.

Mercedes-Benz USA is putting all effort to launch its lukewarm GL-Class now.

The S450 is already available in Europe from €77.256,--- for short
wheelbase and €84.912,--- for long wheelbase. The S450 4Matic are
€80.968,--- and €88.624,--- respectively.

Still no word on short-wheelbase S350 or even S320 CDI for US market.