There is an interesting discussion of Philippians 2:6ff in Gerald Hawthorne's Philippians commentary; he believes that it is erroneous to translate ὃς ἐν μορφῇ θεοῦ ὑπάρχων as a concessive participial phrase (Hawthrone, Philippians, 85).
Hawthorne elects to render the phrase causatively: "precisely because he was in the form of God he reckoned equality with God not as a matter of getting but of giving."
Of course this view seems to presuppose that Jesus is ontologically equal to God and that ἁρπαγμὸν refers to an act of giving as opposed to getting (snatching). So while Hawthorne's proposal is innovative, to say the least, I am not convinced for a number of grammatical reasons.
Firstly, construing ὑπάρχων concessively is more in keeping with the context. If we translate the participial phrase concessively, it illuminates the second part of Phil. 2:6, which reads ἡγήσατο τὸ εἶναι ἴσα θεῷ (Moises Silva, Philippians, 123).
Secondly, within the immediate context of Phil. 2:6a, the humility of Christ is truly emphasized if we construe the participial phrase in a concessive manner. As a matter of fact, Richard A Young also renders this part of the verse concessively:
"Although he existed in the very nature of God" (Young, Intermediate New Testament Greek, 156).
While I agree with Young's rendering of the participial phrase to an extent (since he renders the verse in a concessive manner), I take exception to his translation "the very nature of God" for lexical semantic and theological reasons.
It does not seem that Phil. 2:6 wants to make the claim that Jesus was "equal to God" or bore God's nature. There is an alternate explanation for τὸ εἶναι ἴσα θεῷ. Granted, some exegetes want to interpret the Greek article in τὸ εἶναι ἴσα θεῷ anaphorically, thus they would have the article refer back to ὃς ἐν μορφῇ θεοῦ ὑπάρχων. Yet, grammarian Daniel B. Wallace writes:
"[N. T.] Wright argues that the article is anaphoric, referring back to μορφῇ θεοῦ. As attractive as this view may be theologically, it has a weak basis grammatically. The infinitive is the object and the anarthrous term, ἁρπαγμός, is the complement. The most natural reason for the article with the infinitive is simply to mark it out as the object."
Additionally, P. M. Casey notes:
"On a strict definition of 'incarnation,' Philippians 2:6-11 does not qualify because Jesus was not fully divine, in the view of the original author" (From Jewish Prophet to Gentile God [Cambridge, UK and Louisville, KY: James Clarke and Westminster/John Knox, 1991], 112-114).
While the NIV translates Phil. 2:6, "Who being in very nature God," Carolyn Osiek believes that this rendering is not wholly faithful to the Greek text. Contra the NIV, she does not think 2:6 teaches the absolute Deity of Christ (See Osiek 2000:60ff).