If you currently have GSM service here in America, ask your service provider about it. In fact most companies have some sort of rental program for traveling abroad. (I've worked in the cell phone business for 3 years)
Maybe some of the users here that live in Spain can offer more tips on phone use. I do know that they do have phones for sale with pre-paid airtime loaded on them that you can buy in Spain. During my time in Spain, I found the cheapest rate to be with a callign card bought there in Spain.
Maybe you can find more information on the net or from someone here.