I'm using the WordPress 3.5 Release Candidate version. I did a subdirectory install of WP (in folder WPFOLDER) and then turned on the Multisite mode and created a child site in the network. Both the "homesite" and "childsite" WordPress sites were accessible at their respective URLs, e.g.,
http://www.homesite.com
http://www.homesite.com/childsite
(BTW, "homesite" and "childsite" are merely sample names.)
No problem at this point. I then followed the instructions for the MU Domain Parking plugin (adding childsite as a Parked Domain with my host) and then was also successfully able to access the child WordPress site via the URL:
http://www.childsite.com
Everything seems nice at this point. BUT, with the MU Domain Parking plugin activated, I can't access my home site at:
http://www.homesite.com
due to an infinite redirection problem. Firefox says: "Firefox has detected that the server is redirecting the request for this address in a way that will never complete" and returns the URL:
http://www.homesite.com/WPFOLDER/http://www.homesite.com
Note: I never have an issue accessing the WP Dashboard for the 2 sites via the URLS:
http://www.homesite.com/wp-admin/
http://www.homesite.com/childsite/wp-admin/
If I deactivate the MU Domain Parking plugin, then I can again access my WordPress site at http://www.homesite.com with no problem.
Given that the issue starts and stops as I activate/deactivate the plugin, it seems to be a plugin issue.
http://wordpress.org/extend/plugins/wordpress-mu-domain-mapping/