This works for me on GF6600 and GF9600GT, also with gfxpayload=keep.

I think this bug may be closed.

Oh and add insmod vbe into menuentry

here it is...

I was trying to activate the framebuffer after my grub2 installation.

With the following menuentry it worked, but with just 1024x768 as resolution:

menuentry "Gentoo GNU/Linux, Linux 2.6.30-gentoo-r4" {
insmod ext2
set root=(hd0,1)
search --no-floppy --fs-uuid --set 868e7b2d-ad98-4f62-927e-aaceae9073ce
   gfxpayload=1024x768x32
linux /kernel-2.6.30-gentoo-r4 root=/dev/sda3 ro 
}

If I change the resolution to 1680x1050x32, there is a real resolution of 640x480 and a brightness of about 50%.
If I use linux16 and vga=0x369 it worked like it did before (grub1).
I am having a nvidia graphic card:
01:00.0 VGA compatible controller: nVidia Corporation Device 0644 (rev a1) (prog-if 00 [VGA controller])
Subsystem: Micro-Star International Co., Ltd. Device 1331
Flags: bus master, fast devsel, latency 0, IRQ 16
Memory at fa000000 (32-bit, non-prefetchable) [size=16M]
Memory at d0000000 (64-bit, prefetchable) [size=256M]
Memory at f8000000 (64-bit, non-prefetchable) [size=32M]
I/O ports at df00 [size=128]
[virtual] Expansion ROM at fb000000 [disabled] [size=512K]
Capabilities: [60] Power Management version 3
Capabilities: [68] Message Signalled Interrupts: Mask- 64bit+ Count=1/1 Enable-
Capabilities: [78] Express Endpoint, MSI 00
Capabilities: [b4] Vendor Specific Information <?>
Kernel driver in use: nvidia
Kernel modules: nvidia