I'm running into a problem with Domain Mapper and a plugin called "WP Easy Columns". I've done a little searching in the support forums and it looks like there are other plugins having similar issues.
If Domain Mapping's option to "redirect administration pages to site's original domain" is not checked, then some plugins seem to generate the error "You are not allowed to be here." when attempting to use them.
If any of the developers for the Domain Mapping plugin could take a look at this, that would be greatly appreciated.