Yep, the latest version of Photoshop, (Photoshop CS), now automatically detects US currency and prevents you from manipulating, copying and pasting it.
Paint Shop Pro 8 has also followed suit.
This handy site contains links and facts about using bank note images for countries around the world. And if I’ve read the US guidelines correctly, you can use illustrations of US currency provided you follow these guidelines.
So why is Adobe restricting law abiding Photoshop users?