I have been told that the T410 at least had an option in the BIOS to switch to the discrete card without Optimus as Lenovo did implement a hardware multiplexer in addition to Optimus. T420s might also do this, you should check. One of the Dell Latitudes also implemented a hardware multiplexer in addition to Optimus and thus people have been able to get the Nvidia card running under Linux though without switching ability (but thats still vastly better than no card).

I have been waiting for AMD Llano based laptops as I will be able to run them without any worries about switchable graphics. I need OpenCL and OpenGL support for work purposes.