JSR 275 seems to be in a rejected state. JScience seems to have an implementation and there seems to be unitsofmeasure.
Since the JSR-275 has been rejected the javax namespace cannot be used and has been replaced by the namespace org.unitsofmeasurement. The JScience implementation has been released, latest as of this writing is version 4.3.1 in Oct 2012. Cheers, Jean-Marie Dautelle (JScience project owner)