Community member Kim Parsell recently sent me an email that contained a link to a post she found which describes the exact same problem I was having with the Filosofo Comment Preview plugin.
While doing research on the error, I found out it had something to do with pluggable.php. So, I uploaded a fresh pluggable.php file from a fresh WordPress 2.7 installation and that didn’t work. I ended up just leaving the plugin disabled and was going to work on it on my local server until Kim sent me this email.
Saphod.com publishes a block of code which you can add to the comment preview plugin which should enable both plugins to be activated at the same time. As for myself, I simply disabled Maintenance Mode as I don’t have a need for it since the site is now open to the public.