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.

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.