Domain Mapping - White Screen

I've set up WPMU as a directory install instead of a subdomain install. I've set up domain mapping (which works fine with directory installs as I understand it). I've created a site (http://mystartersite.com/dnelson/) and pointed dannynelson.com to it. The A record update has propagated to me, but all I see is a white screen. I don't see any explanation for this in the Usage guide for the plugin.

Thanks for any help you can provide,

Danny Nelson