After updating to w3tc .9.2.1 and WP 3.1.3, CSS minify no longer works. The minified file just says, "Bad file param format". Since no CSS is being loaded, the layout completely breaks. HTML minify seems to still work fine.
At first I thought everything was fine, but I was logged in so I was seeing the non-cached version. Make sure to check your site when you're not logged in.