I help run a WP multisite installation that has a number of mapped domains using this plugin. We just found out through one of these that password protected posts/pages do not work in conjunction with this plugin (but work fine on unmapped domains). When you enter the password on the password protected page, it redirects to a bad URL in this form: http://YOUR COMPUTER'S IP ADDY/$?action=postpass
We're running the most recent version of the plugin and have (always had) these settings selected in Network Admin>Settings>Domain Mapping: Remote Login, Permanent redirect (better for your blogger's pagerank), Redirect administration pages to site's original domain (remote login disabled if this redirect is disabled). (That is, we have options 1,2,4 selected, but not 3 or 5).
Thanks!