Andrea to the rescue with a solution here by editing the blog address under Site Admin. Works for me.
http://mu.wordpress.org/forums/topic.php?id=9790

A suggestion for future releases: the plugin should update the blog address setting so it doesn’t require manual intervention. It should also revert back the setting if the user deletes the mapped domain. But then what happens with their image links ? Those will be broken as images uploaded with the mapped domain will have the url of that domain ..not the subdomain. This is still the first release, so I’m sure we’ll get fixed up good.