that post got me thinking. would such change in geometry change the way a string vibrates? if so, that would cause predictable changes in the properties of a particle spun arund an axis that runs through the center of the string and perpendicular to the plane on which it exists. so, spin a bunch of particles and eventually you will get one to spin in the correct manner. if you find the predicted change in particle properties, wouldn't that be evidence for string theory?

I'm not sure about the strings, but I know there are some problems in defining rigid bodies in special relativity. I think you have to pinpoint the exact location from where did the circle began spinning (what point of the circle is the first one to experience force) and that would be complicated. I had this problem in my course in electrodynamics, but as I remember we "solved" it by stating that there is a problem of defining a rigid body. Maybe someone else knows ?

However, the QM spin is magnetic moment of a particle, therefore it's not related with the shape, but with EM properties (of "strings", if you like ).

I don't know about that. I heard that spinning bodies aren't well defined in context of general relativity, as in the Riemann geometry the Ricci curvation tensor must be symmetric; this causes problems with exchange of orbital angular momentum and spin because the momentum tensor shouldn't be symmetric in such cases.

One also has Ramond and Neveu-Schwarz sectors which play a part in determing the spin associated with open and closed strings.

I have a rather weak knowledge of GR (and it don't contain many-body or rigid systems), so I don't know. The problem in SR is in the fact that different parts of the rigid body do not (in general cases) accelerate simultaneously.