Vaughan and Gideon's Greek Grammar states (p.79): "An example of the qualitative or characterizing significance of the anarthrous noun is theos in John 1:1." (p.84) [re:John 1:1] "Had the article been used with theos, the suggestion would have been that the Word is identical with the entire essence of the Deity. As it actually stands, theos (without the article) is the predicate, and it is the nature and attributes of Deity that are ascribed to the Word. That is to say, it is the nature of the Word, not the identity of his Person, to which attention is called by the absence of the article."
(To those who would argue that Colwell's "rule" precludes a definite article in the predicate nominative preceding the subject, please see John 15:1b and consider that Greek MS Codex L has a definite article in the predicate nominative at John 1:1c.)
The Zuercher Bibel and Hermann Menge's translation (both German language) have a footnote at John 1:1 noting that the statement signifies that the Word was "goettlichen Wesens" (of the divine essence or nature).
I would be inclined to translate John 1:1bc "and the Word was with God, and the Word was god" because "God" (the definite theos) is construed in English as a proper noun, whereas "god" (the anarthrous theos) is not. (Of course, this is not distinctive when read orally.)
David Bercot (I believe it was he) gave an illustrative parallel using the Hebrew word adam as a proper name (Adam) and qualitatively (adam = human) to illustrate this:
"In the Beginning was Eve, and Eve was with Adam, and Eve was adam (i.e. human)"