According to someone on another forum where the question is also being discussed, holders of foreign passports are not required to have a Turkish driving licence when driving in Turkey, provided that their foreign driving licence is valid in the country in which it was issued.
However, if you have duel nationality, with one of those nationalities being Turkish, or Turkish citizenship, then you will need a Turkish driving licence.
Apparently, this information has been supplied to the poster by a former officer in the Mugla Trafik Polis after checking the April 2008 regulations.