  We study spin interactions between two moving D-branes using the
Green-Schwarz formalism of boundary states. We focus our attention on the
leading terms for small velocities v, of the form v^{4-n}/r^{7-p+n}
(v^{2-n}/r^{3-p+n}) for p-p (p-p+4) systems, with 16 (8) supercharges. In
analogy with standard G-S computations of massless four-point one-loop
amplitudes in Type I theory, the above terms are governed purely by zero modes,
massive states contributions cancelling as expected by the residual
supersymmetry. This implies the scale invariance of these leading spin-effects,
supporting the relevant matrix model descriptions of supergravity interactions;
in this context, we also discuss similar results for more general brane
configurations. We then give a field theory interpretation of our results, that
allows in particular to deduce the gyromagnetic ratio g=1 and the presence of a
quadrupole moment for D0-branes.
