Wow, totally the wrong forum for this post.
I'm no expert on the framebuffer, but I don't believe that X generally uses it to communicate with the kernel. X video drivers are kernel modules just like the framebuffer module so they probably communicate in much the same way. There is a framebuffer driver for X, so I think you can
use it if you want. Generally fb is used for high res console output (not sure you ever see this in Ubuntu, but it's what enables that pretty splash screen while booting too).
In any case, assuming your question is in relation to getting the NVidia drivers working, it's unlikely that the framebuffer is your problem. It would probably help if you could say exactly what is happening when you try to use them. It would also help if a mod could move this to the NVidia Linux forum.