I have a Lenovo Thinkpad t420 coming in the mail soon. I opted for the version with the Nvidia Optimus technology which switches between an Nvidia 4200m (?) graphics card and the Intel HD 3000 integrated graphics. The Linux support for Optimus isn't entirely there, but I've found a couple interesting fixes.
Basically you can either (from the BIOS) turn off the Nvidia card completely or turn off the Intel HD Graphics completely.
A better option (in my opinion) is the Ironhide project which allows you to launch applications using the Nvidia card and uses the Intel HD Graphics for everything else.
Basically I'm keeping some notes in the bottom of this post and will write a tutorial and review later for Linux on the t420 with Optimus.
Ironhide for Ubuntu
Ubuntu Forums Ironhide how-to
Ironhide PPA Ubuntu
Archlinux AUR search for "Bumblebee" : http://aur.archlinux.org/packages.php?O=0&K=bumblebee&do_Search=Go
General Thinkpad / Notebook tips and scripts
ArchWiki - T420
CPU Frequency Scaling