The cell phone registration law allows foreigners to bring in a cell phone as long as it is registered. The problem is--I've never met anyone who can tell me where to register a phone I already have.
As a practical matter, you can use a non-Turkish cellphone with a non-Turkish SIM card on roaming with no trouble (other than a higher cost) for at least a few weeks. After a while, it won't let you connect to a Turkish cell phone provider anymore, but as long as you aren't staying in Turkey for more than a few weeks, you're fine. We've spent up to a month on our Mobal SIM and it worked fine.
If you plan to make a lot of calls, you're better off buying a Turkish phone and SIM and they will register it for you when you buy it. Ask for a "cep telefonu" which means "pocket phone"--they are ubiquitous in Turkey, even in the rural areas of Western Turkey, and cell coverage is excellent there.