I know that nvidiafb doesn't work with nvidia drivers for X, but I don't use X. I just wanted to clear that up right from the start.
I can't get nvidiafb to load a framebuffer. I'm running Gentoo on a Dell Inspiron 1720 with an nvidia 8600M. It runs fine and framebuffers with uvesafb. I wanted to get nvidiafb working for acceleration, and because I'm having issues with mplayer in framebuffer (no point getting mplayer working with 1 framebuffer if I'm going to be switching it).
For uvesafb to work, I had to compile a kernel with initramfs and userspace linker support, then compile/install klibc and v86d, link my kernel to the v86d initramfs and recompile. When I switched to nvidiafb, I left everything the same except I unselected uvesafb support under the framebuffer drivers section and selected nvidiafb support, then recompiled without making any other changes. I copied my bzImage to /boot and changed my grub.conf from video=uvesafb:1920x...... to video=nvifiafb:1920x...., without making any other changes to anything.
I've seen a lot of posts of people getting it to work using the vga= flag, but they all seem to have VGA or VESA support also added to their kernel, and as I understand it, at that point they aren't using the nvidiafb at all and are just using the VESA or VGA ones.
I'd REALLY like to get nvidiafb working because I'd REALLY like to be able to play video from my framebuffer. I don't have a need for X because 99% of the stuff I do is text based. The only stuff that isn't I can easily handle with a framebuffer.