Powerbook waking up but not login window

I hear the laptop waking up (dvd-rw reacting) and see the spinning pizza but I don't get a LoginWindow.
Konstantinos Agouros wrote on :

Hi,

I have my OSX (latest build) Powerbook configured to prompt me for a login after it slept. However in some situations when I shut the lid and reopen it, I hear the laptop waking up (dvd-rw reacting) and see the spinning pizza but I don't get a LoginWindow.

Anybody has a clue for this?

Konstantin