I am using a domain mapping plugin and have my permalnks set to post name. On the main site, WP does insert the word /blog/ between the domain and post name, but only on the main site..
That is expected behavior with multisite.
It is unrelated to theme support. Please post in the wordpress forums if you need guidance on how to remove this. The original poster also uses our sister company, Synthesis, for hosting, which is why I knew some of her specific situation.
These forums were CLOSED to new posts on November 19th, 2012. New forums are here: http://www.studiopress.com/forums/
and you will need to SIGN UP AGAIN.
If you need assitance with setting up your theme, go to http://my.studiopress.com/
and read the SETUP links or the TUTORIALS link on the side tab.
If you need Account Assistance
, file a help desk ticket at the same location.