From Time:

[A]s the prevalence of drug-dusted dollars conveys, it isn't just people snorting and selling cocaine who might have some residue in their wallets. Once a small amount of the substance is introduced, it can spread among bills as they intermix in cash registers, day to day transactions, and bank counting machines, meaning, if you live in a city, you've probably had a laced bill mingling with your money at some point.

The Time article goes on to give this incorrect advice:

Yet, don't worry, you're not likely to face any legal trouble or fail any company drug tests as a result: the amounts of cocaine found on bills ranged from a minuscule .006 micrograms to 1,240 micrograms—an amount comparable in weight to about 50 grains of sand, according to the researchers.

It should have added the caveat: Unless you're charged with a cocaine offense or the Government is seeking to forfeit your property. In that case, you can bet the Government will try to introduce evidence that money in your pocket contained cocaine residue, particularly if a dog sniffed it out.

These studies have been around since the 80's, and despite some courts finding there's no relevance, prosecutors said they'll continue to try and make the connection.

They have, and even in May, 2009, a Maryland court agreed the evidence could come in (Opinion here.)

Because we decline the defendants' invitation to take judicial notice of the fact that nearly all currency contains detectable traces of illegal narcotics, we consider the dog alert evidence as only another piece of evidence tending to show that Carr and Cardona knew that the money involved in the conspiracy was derived from illegal drug trafficking. ... Furthermore, this court has recognized that a district court has discretion to admit a trained dog's alert to currency as evidence of guilt.

The Maryland court quoted a First Circuit case from 1995, that held:

Even though widespread contamination of currency plainly lessens the impact of dog sniff evidence, a trained dog's alert still retains some probative value. Ordinary experience suggests that currency used to purchase narcotics is more likely than other currency to have come into contact with drugs.

(As if the reverse is true, that currency with cocaine is more likely to be used to purchase drugs?)

The Maryland court agreed with the First Circuit case and concluded:

We agree with this analysis, and therefore hold that -- at this point in time -- it cannot be judicially noticed that currency contamination is so widespread as to require the exclusion of canine scan evidence.

The 9th Circuit, even in 1994, had a much better view:

If greater than seventy-five percent of all circulated currency in Los Angeles is contaminated with drug residue, it is extremely likely a narcotics detection dog will positively alert when presented with a large sum of currency from that area. Given this high degree of certainty, the probative value of a positive dog alert in currency forfeiture cases in Los Angeles is significantly diminished and "the continued reliance of courts and law enforcement officers on [such an alert] to separate 'legitimate' currency from 'drug-connected' currency is logically indefensible.

Like I said, time to dust off those motions to suppress.