Actually two of us have I believe. Pulp Fiction was the first to do it and then I did mine.
http://avic411.com/forum/viewtopic.php?t=685
Mine is the Lincoln logo on a burl wood background. Looks pretty cool.
But it's not easy and not free. You have to unlock your drive (and not lose any data on it) and then disable locking on the drive, and there is a boot bmp on the first partition of the drive, just replace that and you have a custom boot image.
To unlock the drive it costs $50 with the software that we found to not kill the data. But doing this is dangerous and can result in a Z1 shaped doorstop. There are procedures if you are real good with PC things and want to try it.
Right now, all we have been able to do it change the boot image, add images to the default directory.
Only Pulp Fiction has been able to image his drive and run off a second drive, mine locks up a lot if I use a different drive. :( But at least I have a custom boot image! :)