Are there plans to fix this? I just set up a WP 3.5.2 MU system from scratch, on a virtualized linux instance in EC2. I had the log-in loop problem even before installing the MU domain mappint plugin. The fix prior to that had been to include the define(COOKIE) directive. That fixed it, but after installing MU domain mapping, the plugin makes me comment out that line. Now I’m back to the same problem. Can’t log into non-primary sites. This thread seems to have gone dormant. Put the problem persists as evidenced by my fresh “from-scratch” setup.