Update: AT&T's site also shows an interesting and possibly telling quote: "While using data on iPhone is free within the U.S., Puerto Rico, and the U.S. Virgin Islands, International data roaming can get expensive quickly. Consider that just 20 minutes of iTunes radio takes 20MB of data. That's why AT&T has created two iPhone International packages with more reasonable rates in 29 countries." (Emphasis ours.) So, iTunes radio, eh? Certainly AT&T isn't talking about downloading tracks over the iTunes WiFi store -- that's only possible via WiFi. Maybe we have something here. Thanks, Ryan.
Update 2: Bonus -- we have confirmation from AT&T that it's not contractual, meaning you can sign up for the Data Global Plan before you hit up a trip and then drop it when you return. We don't have pro-rating details, but we're sure you'll work it out.