What you originally did seems right. And no, you can't have any other entries for blog.mindshare.com or "DNS will get confused".
Did blog.mymindshare.com point to something else before? On my machine it resolves to 220.127.116.11 and is on a typepad server - it is a blog and has the nice yin/yang icon and says it's the mymindshare blog.
It is different from the blog that's at jimbursch.typepad.com.
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