As mentioned, It may be possible to recover some erased ones, but you must quit using the Computer immediately, because the OS will think it can write to those freed blocks on the disk.
Either boot this one in Target mode...
http://docs.info.apple.com/article.html?artnum=58583
And recovering from another Mac, or booting this Mac from another HDD, then using Data Rescue...
http://www.prosofteng.com/products/data_rescue.php
(Has a Free Demo to see if it could or not, but you'll need another drive to recover to).