I have read the posts above and I'll try now to give my opinion on the matter. Before everything else I must stress something: I have read in the past the articles of the Hellenic Constitutions from the times of Otto's reign which dealt with the royals and they are perhaps
the most imcomplete constitutions ever in these matters - they don't even clarify the line of succesion, whether it is semi salic or male primogeniture. Also , I don't remember to have ever read one single article in any Hellenic Constitution that stated any reagulation about who is a and Prince/Princess of Greece , any criteria of the matters of spouse or even that members of the family need permission to marry from the Head of the House in order to marry and retain their succession rights and titles- AFAIK there has only been one legal decision about this, regarding the marriage of King Alexander and Aspasia Manos, where the court decide that the marriage between them was perfectly legal , but when it came to titles and the line of succesion, the one who should decide about titles and the succesion was the King, and it has been like that ever since, with the King alwas acting as he pleased .
In three cases I know, titles did exist even without any or with questionable succesion rights
a) Alexandra, the daughter of King Alexander and Aspasia who was never eligible for the Throne but was a Princess
b) Peter ,son of Prince George and Marie Bonaparte, whom the whole Greek Dynasty practically renounced after his marriage to a Russian commoner, but was still referred as a Prince ,inside and outside the country
c) Michael , who renounced his succesion rights before marrying but is still a Prince, and the lasrt time I checked was on the website of the GRF (his daughter however are not)
In the case of Philip I honestly don't know what happened. I have seen a scanned paper on the Net about this matter and I also posted it here somewhere.What I understand though is that King George II was able to decide on the matter, without any Greek laws to place boundaries, not even stating what it would take to make the renounciation valid. And that following precedent, perhaps Philip could continue to use his titles if he wished (I don't know if renouncing his nationality plays a part though)
BTW, Philip and his descendants are definately not eligible to succeed on the Greek throne regardless on any valid or invalid renounciations, because they don't fill 2 of the 3 constitutional conditions : a) Having Greek nationality and b) being Member of the Eastern Orthodox Church.
PS At the time of the marriage between Philip and Elizabeth, the Greek royals asked Spyridon Markezinis to investigate all the legal aspects of the matter. Markezinis wrote many books about the history of Greece , with reference on the Greek royals so perhaps he also wrote something on the matter. Ill try to check it during the holidays if there's anything interesting thers
EDIT: Here is the scanned paper I reffered to , I see it mentions nothing about titles
http://www.styx.gr/photos/eggrafoadeias.gif