It seems somewhat counter-intuitive that causing fewer customer delays (and less aggrevation) and reducing your staffing needs could have a negative effect on your business, but that's exactly what this article at Network World is suggesting. Specifically, according to a study by IHL Consulting Group, they report that "consumers report buying junk food, supermarket tabloids and the like 45% less frequently while scanning their own purchases than when checking out the old-fashioned way."
Considering that almost a fifth of consumers report using self-checkout every time it's available (and that's a 35% increase over 2004 numbers), at-retail marketers will have to work hard if they want to retain the same number of impulse purchase dollars being spent at checkout. And while a decrease in the total amount spent on these purchases isn't likely to stop retailers from deploying more self-service solutions, it may give pause to some who were having trouble deciding on whether to implement the solution or not.
Update: Here's a link to a more complete version of the story.