That's significantly worse from a user perspective than giving Amazon a credit card number to enable one click checkout or downloading a book from my kindle. It's true you could do anonymous transactions online, but while it's better than mailing people cash it's something of an edge case and I could also buy a Visa gift card and get the same sort of anonymity. Again, I like Bitcoin, but the problem IMO is how to make it both as convenient as a credit card and secure.

PS: Your also describing an adhock solution. As soon as you want to mass produce them to allow significant and convenient adoption you get into regulation issues. And by 'device' I am including just the software to manage your account from a cheap netbook.