Here's my take on it:
The US Secret Service guide to
detecting counterfeit currency doesn't mention the detector pens at all. Whether this is significant or not is debatable, but I would think that if they worked they would say something about it. But I could be wrong.
Dri-Mark, a manufacturer of the pens,
says:
IMPORTANT: While no method of counterfeit detection is perfect, the SMART MONEY™ is an excellent deterrent to the passing of counterfeit U.S. currency and will detect a great majority of false bills. We recommend this product be used in conjunction with other detection methods. The SMART MONEY™ will not detect any series printed before 1959.
If the great majority of false bills are printed on starch-laden paper, then this statement may well be true. But cotton fiber paper which, to my knowledge, will pass the test, can be bought for under $20 for 100 sheets, and the only data I can find with admittedly very limited digging says that computer generated bills accounted for less than half of all counterfeit bills in 2000 (
here).
The problem I see is that the pens are being used instead of, rather than in addition to, other detection methods. Sites like
HowStuffWorks.com stress that the cashier needs no training to use the pen and
howstuffworks.com said:
"All of these [security] features are nice, but no store clerk is going to stand and hold each $20 bill he or she receives up to the light to check for a security strip! It takes too long and it is not a flattering pose to strike...
and
techtarget.com said:
Detection pens are easy to use and require no training. A clerk at a cash register simply uses their counterfeit detector pen to put a small mark on the bill. If the bill is counterfeit and the paper is wood-based, the iodine in the pen solution will react with the starch and leave a dark brown or black mark. If the bill is authentic and the paper is fiber-based, there won't be any starch and the pen will not leave a mark.[emphasis mine]
This statement should actually read "If the bill is authentic
or the paper is fiber based..." or, rather, just "if the paper is fiber based..."
And, of course, I have found nothing about the rate of false positives caused by genuine bills being contaminated with starch (with or without Randi's help

).