The bill included $114 in data roaming charges. (!!)
Fortunately, I'd seen a blog post in the last week or so, with the subject "Goodwill", talking about a similar situation -- the poster had made a weekend trip to Victoria BC and managed to rack up a $300 bill. When they called AT&T to complain, they managed to talk with a phone rep for like an hour before the rep happened to mention "oh, there's this thing I could do that would knock that charge down to $25." The poster was saying that treatment like this will cause AT&T to lose customers in droves when the iPhone is finally no longer exclusive to them, but thanks to that post, I knew to call AT&T and ask for that retroactive change.
It took quite a while -- the rep I got was ignorant but very helpful -- but I did eventually get that $114 charge reduced to $25. The magic incantation is "Data Global Plan" and you can find more information about it here: http://www.wireless.att.com/learn/international/roaming/iphone-travel-tips.jsp. You can be sure I will check back next month to make sure we don't get charged the same $25 every month going forward.
So the only remaining question is: who was that masked poster? I've Googled all over and can't find the original post.
ETA: An anonymous commenter tells me that the original post was here: http://zzzorg.blogspot.com/2009/08/how-to-lose-customers.html. Thanks to The Zorg for letting me know this option existed! If not for that I would have just sighed and paid the $114.