Thanks, David (rachfan) and emill, for your kind words. Roy (yes, he has a doc day job, like me) plays a mean fiddle, no? Apparently Martinu was circumspect about Einstein's violin playing at the premier performance on campus at Princeton. Also, the composer had a deal: Einstein was supposed to explain his theory of relativity to him in exchange for the piece dedication, but Martinu never could figure it out, even with the physicist's help.
Of course, there is also the recurrent and possibly apocryphal story of someone (Rubenstein? Schnabel? Heifitz? Casadesus?) playing with Einstein, having him lose his place, and complaining, "Albert, can't you count?"