I have just now encountered the same problem, I think.
I had installed Tiger using Archive and Install / Preserve User Settings. I have been using it for a more than a day now. I have been able to successfully restart my 1GHz Powerbook G4 several times.
Just now, I was trying to restart, and iTunes wouldn't exit. Force quits didn't work. So I had to hold down the power button until it did a hard shutdown.
Now, whenever I type my password into the GUI login screen, it goes blank, then drops to Darwin/BSD console mode. It then prompts me to login with username/password, which I can do and it puts me into my home directory. But I'm in console mode!
How do I get back to using the GUI?
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UPDATE: I booted from the Tiger install DVD and did an upgrade install so I wouldn't lose anything, and it seems to have fixed the problem. Shew.