I would probably just move all applications to mod_rails, as the performance seems comparable to Mongrel and there are less administrative overhead.
Regarding gotchas configuration, just make sure you have resolved your shared directory or you will find that static assets are not working:
<Directory "/var/www/app/current/public"> Options FollowSymLinks AllowOverride None Order allow,deny Allow from all </Directory>
Also, if you know how to configure Apache, mod_rails is very painless.
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