I believe we have have a T420 with switchable graphics in our lab. I know we have used it in testing in the past and have had no problems with it.
Let me go run a few tests with KVM and I will report back what I find.
So I just finished testing the integrated/switchable graphics setup on our T420 in the lab.
If you have the system set to use Nvidia Optimus in the BIOS, KVM works great.
We ran an Nvidia demo off of their site, making sure the discreet card was activated, then we used VNC+ to KVM into the system.
It worked great and allowed us to see and interact with the full screen 3d demo (being run on the discreet GPU).
We then went into BIOS and selected only to use Discreet and attempted to connect with KVM.
VNC+ refused to connect to the system.
So as long as you are running with Optimus or Integrated, you should be able to take advantage of KVM.
All of the other AMT features work great on that system as well.
hope this helps!