thank you for the so valuable information. I had the same problem and i've left my macbook pro on the side for a couple of months (apple told me it's too old and cannot support it, it's 2006 model).

So, i followed the instruction of using the: /sbin/fsck_hfs -ypr /dev/partition
and it worked fine after only i did boot from the OS cd and only from the terminal.

I run it first from single-user mode, but nothing happened.
Booting from the OS X cd, i tried to repair the disk from the Disk utility, but again i got the message that the drive cannot be repaired and should format it and install again the OS.

So last try was to run the command from the terminal, and it worked fine!
I've my favourite mac back in business again.

Thank you guys alot for your help!