Basically, we call native accept() and that's non-interruptible. For other sockets, we use java-based infrastructure and do tricks with select there, but for Unix Socket, we go right to the native calls.
Looks like MRI (and Python for that matter) do clever tricks when they implement accept for sockets. They set the socket to O_NONBLOCK mode, and do select() until the socket is available, or something like that.
For Python, the code is: http://svn.python.org/projects/python/trunk/Modules/socketmodule.c (internal_select and friends).