According to the Thinkwiki for the Thinkpad T420, it has either:
- Intel Integrated HD Graphics 3000, or
- Discrete NVIDIA NVS 4200M (1GB VRAM) with Optimus™ Technology
jdmcdaniel3 gave (IMHO) some good advice.
Which does yours have ? Or does it have both ? If it has both it is a hybrid graphics device, and we are into a completely different 'kettle of fish' in that case.
Can you please run the command in a terminal:
and post here the output ? That will tell us what graphics this T420 of yours has.
/sbin/lspci -nnk | grep VGA -A2
If yours has only the NVIDIA NVS 4200M graphics, then it is possible the 'nouveau' driver does not yet support it properly, and you need in such a case to turn of Kernel Mode Setting (KMS) which you can do by specifying the boot code 'nomodeset' as noted by jdmcdaniel3. That will boot to the 'nv' graphic driver (which is of poorer performance than the 'nouveau' driver). That will only be a temporary solution, and in that case for the NVIDIA NVS 4200M ultimately you will likely need to install the proprietary graphic driver (referred to as the 'nvidia' graphic driver). If 'nomodeset' does not work you can also try 'x11failsafe' by itself, or in together with 'nomodeset' (ie 'x11failsafe nomodeset' as a boot code) which will possibly force the PC to load the very basic FBDEV driver. Again, that is only a temporary solution until the proprietary 'nvidia' driver is loaded. I note you have discovered 'acpi=off' and 'maxcpus=0' also work for you, and instead you could use those as an interim until the proprietary 'nvidia' driver is installed and loaded.
However if your T420 has the Intel Integrated HD Graphics 3000 then it should 'just work' without the need for 'nomodeset' or 'x11failsafe' or 'maxcpus=0' or 'acipi=off'. I have read of some GNU/Linux users finding for Intel Integrated HD Graphics 3000 that the boot code "i915.semaphores=1" works. ... And finally if you have the "Intel Integrated HD Graphics 3000" , I think that is also known as 'Sandybridge' graphics, and for that you may need to wait until openSUSE-12.1 GM is released (and until it is debugged as the 12.1 beta is having a rough time currently). I think SeanMc98 has pioneered an interesting technique for "Intel Integrated HD Graphics 3000" that requires no boot codes .... (although I may have the Intel hardware wrong). In his case he simply opens and closes the laptop screen at a key point during the boot process, which enables the laptop to nominally work (although based on your description of the 'freeze' location your problem reads to be different).
And finally, if this laptop has hybrid graphics (ie both Intel and nVidia) you may need to turn one off in BIOS (as noted by jdmcdaniel3) until you can apply a possible approach such as provided by the application 'bumblebee'.