In an attempt to buy Weather Forecast Widgets this weekend, I discovered that American Express can't be used for foreign currency purchases through the Market (though I've never had a problem with it elsewhere).
I also find that I'm unable to download the application. It is finally "authorized" but I always get a "download unsuccessful" when I attempt to download it. I've tried clearing the cache and data from the download manager, market, and market updater, then rebooting, but that did nothing. What do I have to do to get this installed? Hopefully I won't have this issue every time I attempt to purchase an app. Free downloads have never had any problem.