Merriam-Webster sez:

Main Entry: West�pha�lia
Pronunciation: west-'fAl-y&, -'fA-lE-&
Variant(s): or German West�fa�len  /"vest-'f�-l&n/
Usage: geographical name
region W Germany bordering on Netherlands E of the Rhine; includes Ruhr
valley; a province of Prussia 1816-1945 capital M�nster -- see NORTH
RHINE-WESTPHALIA
- West�pha�lian  /wes(t)-'fAl-y&n, -'fA-lE-&n/ adjective or noun

It appears that Westfalia is the German spelling, while Westphalia is the
English adaptation.  See also http://www.nrw.de/.

Either way, Westfalia is the correct way of spelling the brand, therefore
Westphalia is not acceptable.  Same goes for Volkswagon.

Wow.  Did I just agree with Ben T?  Must be the holiday spirit!  :-)

Chris.