You may want to explore the permutations of using (foreign) pre-paid service on an unlocked quad band GSM/UMTS smartphone w/ wifi while in Spain (and the other countries). Going with a U.S. carrier's international plan will basically cost you an arm and a leg by comparison. It's easy to grab a pre-paid SIM either at the Madrid/Barcelona airport (more expensive) or at a neighborhood kiosk (cheaper). Having a wifi enabled phone would give you access to google voice or perhaps another voip provider that you can call back home on the cheap. (There was actually a decent post/solution on this very topic last year that I will try to dig up.) Anyway, the cost of the pre-paid will be insignificant, you will just have to find wifi for Google voice to take advantage of the free outgoing SMS to U.S. numbers.

http://karhoe.net/free-sms-with-google-voice.html

See if one of your friends can send you an invite for GV.

Also, a lot of students travelling also use Skype for shorter calls back home when cellular. Yet another reason to grab a wifi enabled smartphone, something like either the unlocked/unbranded Nokia E63-2 or E71-2 ($149-299). Check Amazon. The T-Mobile Dash is another useful quad band Windows Mobile phone with wifi that can sometimes be found on the cheap for $99 on fleabay.