C.K. pointed out a post at creativebits.org highlighting the fact that
Photoshop has currency detection built in. This reminded me of a fantastic lecture I attended at the Chaos Computer Club a year ago by Steven J. Murdoch on currency detection systems.
In case you were interested, it's not only Photoshop that has currency detection systems built in (with undisclosed algorithms) but also many printers and scanners. An interesting note from Murdoch's talk at 21C3 was that some printers will even automatically skew certain colors (i.e. orange becomes brown) that appear in bank notes. The creativebits.org article notes a quick workaround if you just HAVE to use money in your, ahem, artwork.
[slides for Steven J. Murdoch's talk can be found here]