it's true. over the years, i've tested a number of poppy seed samples (it's part of my business) for the presence of opium. and as for the results, the phrase YMMV applies. generally, the samples are brought in by individuals that have failed a drug test and want to use the "poppy seed excuse". most are fairly desperate and are willing to pay well for the analysis (more money to hobby). not all samples test positive. one client brought 5 seperate samples. he kept trying until he found one that tested high enough to justify his claim. keep in mind, that if one wants to make this claim, he should be prepared to allow the other side to do their own tests (doesn't always happen but sometimes it does). i have also tested the raw poppy seeds for various importers for the presence of opiates and again the results are YMMV.
I have also done testing for the "Peruvian Herbal Tea excuse".
There is a (or perhaps several) herbal tea available in Peru that incorporates a certain amount of cocoa leaf. this product tests positive for cocaine and individuals with a positive cocaine metabolites test will use the excuse that they consumed this tea prior to the test. up until a few years ago, this tea could be occasionally found in this country but its importation has been since been banned by the FDA/DEA. now individuals have to use an explanation that their aunt/friend/ect. gave it to them as a gift which they acquired while traveling.