Been fighting with this all day. =)
I read through this thread, which helped a little, but didn't solve the problem.
I have MainMultisiteInstall.com. The blogs are all mapped to their own domains using WordPress MU Domain Mapping. This works great.
However, I'm consolidating servers. One of my blogs has subdomain blogs.
I'm trying to map
TopicX.MainMultisiteInstall.com to TopicX.ThisBlog.com
ThisBlog.com is a parked domain. Works perfectly.
TopicX.ThisBlog.com has this CNAME record.
TopicX.MainMultisiteInstall.com. 3600 IN CNAME TopicX.ThisBlog.com
Doesn't work. Blank page with a directory index.