When I place my X220 on a dock that has ethernet connected, it does not switch to wired connection, instead it stays on wireless.
Is there anyway to enable this auto switching functionality? The wifi at the office is open, I don't want to keep connected to that (public) instead when I dock I want to go through our local Ethernet without having to switch antenna on and off.

I was able to achieve this by using Access Connections and setting the office as new location. I don't know how it recognizes the new location, but I assume it uses wifi to determine the location first, then switch settings. Now it is successfully connected to LAN.

I don't use Access Connections, but I had the same issue. I'm pretty sure I fixed it just by going into Windows 7 connection manager thing, seeing it was connected to both ethernet and wifi, and disabling the wifi connection. Somehow Windows remembered that as its only connected to ethernet when docked since.

In my case, windows was not remembering it, what happened was it never enabled (or started using) wired connection, even though it was enabled. I think windows uses wired connection as priority, but for some reason it wasn't routing data via wired through dock.
Access Connections is a bit flaky, it hangs when you want to modify settings and save them, but once saved, it runs seamlessly.