I too had hosed my 10.5.6 upgrading (through the automatic Software Update...) to 10.5.7.
After re-booting my screen was unrecognizable, gray and remnant laden.
I was however able to boot into safe mode. I then "restored" the disk (disk utility) to another SSHD (or any HDD) via usb cable.
I ran the Mac OSX install on the Dell Mini9. Rather than a "clean" install I used the Transfer option using the "restored" image, in effect returning my OS to 10.5.5...
I the followed the proper upgrade path using the Combo 10.5.7 (DellEfi steps)...
Seems to be back on track. I will try to keep up with this site in the future!
Hope this helps!!