[Kyle Fenton]

Try using Disk Warrior First
http://www.alsoft.com/DiskWarrior/
This will repair disk structure. As long as there is no mechanical failure this should work.

Otherwise you can use Data Rescue II by Prosoft. I haven't used this by I heard that it is effective.
http://www.prosofteng.com/products/data_rescue.php

You could also remove the internal HD and mount it in a temporary USB enclosure, like this one (http://www.newegg.com/Product/Produc...2E16812156102). From here you can go into the Unix terminal and see if you can force mount it.

Other questions:
Do you hear any clicking or abnormal noises?