  We consider a particle dressed with boundary gravitons in three-dimensional
Minkowski space. The existence of BMS transformations implies that the
particle's wavefunction picks up a Berry phase when subjected to changes of
reference frames that trace a closed path in the asymptotic symmetry group. We
evaluate this phase and show that, for BMS superrotations, it provides a
gravitational generalization of Thomas precession. In principle, such phases
are observable signatures of asymptotic symmetries.
