I've driven on three of my trips to Spain, and have never needed the famed "IDL". With your US driver's license, you'll be fine, and if you can explain to a potential "'I'm taking your driver's license' Cop" that the DL is your ID, I'm sure he'll think the better of it.
Depending on your Spanish skills, you'll have no trouble explaining what happened and how you can fix it (this, assuming that you're stopped...which isn't a likelihood).
I'll be driving again in May and let you know how it goes. As far as renting a car goes, I've used Avis and Alamo, all arranged in the US, and have never needed anything other than the US DL and credit card to complete the rental. When I had a claim against Avis in '99 (two flat tires), they still didn't need any IDL or any other paperwork. Sometimes they don't even check your passport.
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Ongi etorri!