In terms of debugging the problem, I haven't worked out what causes the error but I know how to fix it. There seems to be a strange interaction with \DeclareRobustCommand and the xparse package which we'll need to sort out.
Scratch that. Just author stupidity in realscripts.sty. W -------------------------------------------------- Subscriptions, Archive, and List information, etc.: http://tug.org/mailman/listinfo/xetex
