cell phones in Spain

For a 2 week trip I would recommend simply adding an international plan to your cell phone plan. I had perfect coverage with AT&T around the Costa del Sol up to Madrid(I went no further north than Madrid). The only downside is that you are charged per minute, so I kept the talking to a minimum and mainly used text messaging.

Unless you plan on using your phone quite a bit, temporarily adding an international plan should still be cheaper than buying a separate pre-loaded cell phone specifically for use on your trip.

I was in Spain/Portugal for two weeks and just used my AT*T phone with the normal plan. I do think you need to call and find out if they enabled international roaming for you.

If you don't call much it's not that bad. I think my total bill was around $60 and that included one long call where I chewed out Bank of America. (They "Forgot" to note that I was in Spain AFTER I spent an hour on hold to get that done, final blow to that relationship!)

You can either get a SIM card when you get there or get an internation SIM before you leave. If you get the card when you get there you may get a better deal on rates by going through a European or Spanish carrier, but without a permanant address in Spain, you'll be limited to pay as you go by the minute cards.

You can also buy an international SIM before you leave. The rates are often slightly higher, but the rate is locked into many countries for one price. For example, some SIMS will work in all of continental Europe for $0.35 a minute and features free incoming calls from land lines (the person in the US dials and 1-800 number, then your phone number, and they call you for free and you get the call for free) These are good if you're hitting a few countries.

You can get a cheapie phone at any phone store or the airport. They are local and your friend can load on as much money as they want. You can go to the grocery stores (chain type) and "top up" as you want.
You can't save the money you don't use so don't over load up close to departure. Also, when you return in a year or so your number will be shut off due to inactivity. So you'll have to buy a new number.
On one trip we got a pretty decent phone and two years later a kid went to school in France and used it. I am still buying phone numbers for the $35 phone I got there several years ago. I have one for England, Netherlands and France each... Maybe the whole united thing is better but in '05 it was not!
Check with your provider on pricing issues before you go.

Try www.onspanishtime.com
He rents and sells cellphones and has great deals on SIM cards.

I've done business with him on two trips and was very satisified.

You can also get a quad frequency unlocked phone from Amazon (arond $50) and buy a sim card once you arrive (or you can buy them online).

As others mention you can also add an international plan if your phone works in Europe. It will be more money but unless you are going to use it often it will be a minor cost compared to hotels, food, etc.