What is it ?
A framebuffer driver is a piece of code inside the kernel which allows displaying a nice terminal with fancy graphics without X.
Why could it be a problem with Nouveau ?
In order to work, Nouveau (the DRM part) need to own and know the state of the card. A framebuffer driver, especially nvidiafb, can (and will) modify the state of the card by itself, confusing the Nouveau driver if they are run at the same time.
However, operating in very limited mode, some framebuffer driver can be used such as vesafb (x86) or offb (ppc).
How can I disable nvidiafb (or another framebuffer driver) ?
On x86 : suppress video=nvidiafb from your boot command line (lilo.conf, grub/menu.lst) ;
if you want use vesafb, you can append vga=xxx (see VesaFB Documentation)
On PPC : append video=ofonly to your boot command line (or replace another video=xxxx)