Is there a Wi-Fi-only mode? Once I am connected to a Wi-Fi network (regardless of my other connections to 3G and 4G) is there any way to guarantee that all my data goes through Wi-Fi?
Your phone will default to WiFi when it's available.  If you're paranoid you can disable data while using WiFi but it's not necessary.  I'm not sure how to do it on the Bionic but you could see if it's in the menu that pops up when you hold the power button for a couple seconds or something like unchecking Use packet data in Settings -&gt; Wireless and network -&gt; Mobile networks.