I've been tracking pink robinias this year, which has been especially easy the past few weeks, while they've been in flower. Turns out the plants I see in Metrowest Massachusetts are at least two different species: clammy locust (Robinia viscosa) and bristly locust (R. hispida). Along with the white-flowered black locust (R. pseudoacacia), they are considered native to the U.S., but only the southeastern region. Check out a gallery of pink-flowered specimens I've uploaded here. Some of them seem to combine characteristics of the three species, so perhaps there is some hybridization going on.