Journeys (the shoe chain) and Hollister Co. both use Linux distros on their point of sale machines. Hollister's machines are pretty locked down and can basically only run the RPoS software, but a lot of Journeys' software is browser-based, so they have to be a bit more capable.
Pretty sure they're both custom distros, though.
"It is not the severity of punishment that deters crime, but the certainty of punishment."