I have also that problem! My Lion OSX crashed - black screen.
Needed to power it down. After that grey screen and apple logo.
OSX was not starting.
After booting with cmd+V a HDD error was reported.
Repair with Recovery Disk Tool did not work.
Disk Warrior 4 fixed the errors. Now I can boot my OSX until the login screen, bot none of my users can log in!
I repaired the rights, fixed the harddisk.
After typing password and ENTER a short white screen comes up and then back to login page.
None of my 3 Users and also root are working!