I have T-mobile

T-Mobile does offer roaming service from Spain, but it is not inexpensive:

Spain
Rate per minute : $1.29
*Roaming charges do not include local tolls or long distance charges.
Country code : 34
Carrier Frequency Data Speed
Orange 1800Mhz 2G
Telefonica Moviles 900Mhz,1800Mhz 3G
Vodafone 900Mhz,1800Mhz 3G
Xfera Moviles (Yoigo) 3G 2100

Click to expand...

I have an iPhone 3GS with an ATT Plan, and I added the European service before last year's trip. Yes, it was a bit pricy, but it saved hunting up recharge cards for my French cell phone for calls home, and it allowed my family to reach me as they normally would, rather than having to learn about international dialing (worth the price for reassuring ailing and elderly parents).

I think ATT and T-mobile use the same backbone and network technology (they're merging this year). The service worked well for me throughout France. But at a buck a minute, do keep an eye on your call time. And let the home folks know something about the price -- it will let you bring chats with the chatterboxes to a rapid conclusion.