Enabling matroxfb on a stock Ubuntu Feisty is somewhat non-intuitive.
First you have to add the necessary modules to your initrd. I own a Matrox G550 and as I do not know which modules are required I chose to add all of them:

ls /lib/modules/`uname -r`/kernel/drivers/video/matrox | \
sed -e s,\.ko$,, >> /etc/initramfs-tools/modules
update-initramfs -k `uname -r` -u

Enabling matrox framebuffer support using a 1024×768@32 in grub using the matroxfb.txt document found in the kernel documentation (or on google.com). Change the line starting with # defoptions to
# defoptions=quiet splash video=matroxfb:vesa:0x118 and call update-grub
Note: When using usplash I could only use resolutions 640×480, 800×600 and 1024×768. Because only for these resolutions splash images are available in the default usplash theme