We are testing W3 Total Cache with some of our installs (we have about 20 to test) and on the few we've tested with we've come across an issue.
I start by customizing the settings, for our purposes we're only enabling the browser cache and object cache. I export these settings (again, multiple installs this needs to go on) and import them into the next site, the settings import properly (preview automatically enables) and I disable preview - object cache is no longer enabled.
Our install is 3.0.4 with Networking enabled. I have tested and recreated the issue after disabling our other plugins.