I was left in a position where I could no longer sudo to rename it back.  I had no luck from Finder either and I couldn’t open new Terminal sessions.  I decided to reboot in safe mode by holding down shift while rebooting to see if I could repair the damage from there but my system just wouldn’t come up – I was left staring at the Apple logo.

In case you should ever find yourself in this position, don’t panic too much – After much panicking myself, I managed to fix it by doing the following.  Bear in mind at the time of writing I use Leopard so things may be a little different in future OS X versions.

• Locate your OS X installation disk 1 and pop it in the DVD drive.
• Reboot while holding the C key down to force your Mac to boot from the DVD drive.
• It’ll load up and ask you what language you’d like to use.  Pick one (preferably one you can understand lol).
• The next screen is all about reinstalling.  That’s a last resort really because we can fix things from here.  On the Utilities menu you should see Terminal.  Choose it and the Terminal will open with a bash shell.
• Bear in mind that you’ve not booted from your hard disk so the directory structure will look different to what you’re used to.  Change directory in to /Volumes and then in to your hard disk, possibly called “Macintosh HD” unless you’ve changed it.
• Now you’re actually looking at your hard disk and should be able to change directory in to usr/lib and mv the file back to it’s original name.  Just to clarify, you should not be in /usr/lib.  You should be in /Volumes/Macintosh HD/usr/lib.

Good luck!