I searched the archives but hope to get the most up to date answer so am asking it in this post. I have an iPhone 3Gs that I no longer have on a plan in the USA and it is unlocked. I have taken it to the UK and Australia last year and set up a short term plan and purchased a Sim card. I mainly like to have it for some of the apps and the GPS but find it comes in handy if I need to make or cancel a reservation or other calls that occasionally come up.
After a Google search I saw an article on new laws and fees for foreign cell phones that went into effect in October but I am still a bit unclear. Our trip is only 10 days so I am not sure if we will need to register and pay a tax as well as should I buy a Turkish Sim card or use the one already in the phone. I don't plan to make any calls outside of Istanbul or back to the USA and most likely won't make any calls, just mainly use it for the apps as mentioned.