I had two blogs going and one of them had a domain name mapped to it.
I wanted to change it so that mydomain.com be mapped to b.wordpress.com instead of a.wordpress.com.
So I deleted a.wordpress.com which I didn't need anymore and tried to add the domain name under Options->Domains but that didn't work as it gave me the message that the host name was already in use.
So now going to mydomain.com still takes me to a.wordpress.com even though I deleted the blog!!!