This happened to me one time after I was playing with things I probably shouldn't have been in /usr/bin... I ended up archive and installing, then only updating up to 10.5.2. There are a lot of problems 10.5.3 caused with the NVIDIA drivers, so I thought it safest to not install that update again. Before doing something so drastic, though, attempt to boot into safe mode by holding Shift as the machine starts up, and keep on holding it until after the second loud chime. If this does not work, try holding Command+Option+P+R as the machine starts to reset the NVRAM. If this does not work, unplug and remove the battary from your MacBook Pro (after shutting off), leave the machine disconnected for 30 seconds, then press and hold the power button for 5 seconds. Finally, put the battery and power cord back in and start up. This will clear the SCM. If none of this works, boot from your OS X install disc by inserting it while the computer is on, then restarting the computer and holding down C. At the top after you go past the first welcome screen (but don't go through with the whole installation!), there should be a Utilities option. Click that and use Disk Utility to repair your disk before restarting. If none of this works, just go for Archive and Install. It only takes a couple hours. I hope something here helps.