I was in Turkey throughout the summer for some internship and I can truly say that it is such a wonderful country.
I brought my iPad 3G with me and I was able to use it without any problems. I bought a prepaid plan from Vodafone, 5 gb for around $30. And most importantly, they have micro-sim!!! It was pretty easy to get the job done. Somebody above just said that all have micro-sim but I don't think the same. Turkcell didn't have and none of the salesman had any idea about what a micro sim is!
God, I can truly say that At&t sucks compared to the 3G speeds in there.
I don't think you can do that from UK or US, I know that all the carriers in both countries have agreements with Turkish carriers but data rates will be very expensive I suppose. You can do it easily after you got to Turkey. It took me less than 15 minutes to activate it.
If you have any more questions you can pm me..