I don't personally use the T420, rather a Y570, but certainly at the moment, I would only buy Lenovo laptops because they are the only manufacturer, that I know of, that allow you to TURN OFF the Optimus stuff in the bios.
A lot of new machines with this "feature" allow bios swapping between 'inbuilt' mode and 'Optimus' mode. Lenovo actually allow 'discrete mode only' bios swap as well! Therefore, install Nvidia drivers and works like a charm. Doesn't have to know about the Inbuilt graphics. Mine is a desktop replacement so it is Linux only and connected to power most of the time - couldn't care less about Optimus!!
You know it only requires the builder to spend literally a few cents extra in the build process to allow this!
Life isn't about waiting for the storm to pass...
it's about learning to dance in the rain.