How can I restore file permissions to get my Mac booting again?
You should be able to do the following:


Boot up while holding the "Option" key (or command + R for lion and mountain lion users
Since this is a Lion (or later) system, one of your boot options is "Recovery" - pick that
Once the recovery partition boots up, go to the menus and select "Disk Utility"
Find your Macintosh HD drive in the list on the left, select it
Go to the First Aid tab
Select "Repair Disk Permissions"


This will take a little bit, based on the size of the disk, but you'll be up and working. After the permissions are repairs, it's a good practice on HFS+ (your filesystem) to choose "Repair Disk" also.