Blog creation from mapped subdomain

Is it possible to create a new blog from a mapped subsite?

I'm using multisite 3.1 with subdomains and I've enabled Buddypress on all the sites. If a user creates a blog from a subdomain, the resulting url is . That's okay, but I'm also using domain mapping. When a new blog is created from one of the mapped domains, it creates a new site but that site is inaccessible because the resulting url doesn't make sense (e.g. "http://mynewsite.")