Never do this

 sudo mv filename.file /bin/sh  

Just don't mess with your bin/sh, and secondly don't attempt to restart your computer after you mess with your /bin/sh (which is the symbolic link to /bin/dash - low level crazy stuff which ordinarily you shouldnt ever know or care about, but which ensures your computer can do some vital things, like initialise).

This I discovered to my cost, and now I'm having to spend my weekend recovering all my files

This is a worrying and heartbreaking exercise, as I'm sure you'll understand - my computer is my livelihood and most of my hobbies, and I havent backed stuff up for ages, and recently I've been doing some cool stuff which exists only on the computer which now needs a complete OS overhaul!!

I'm writing this for instruction and warning rather than pity, you understand. You can't reboot, not even in any safe mode. I did a bit of internet forum research, and there was little guidance (mostly, instructions for windows users to use Ubuntu to recover their files, but not Ubuntu users to recover their files)

It turns out it's thankfully simple.

1. download an Ubuntu distro iso, and burn it to CD (instructions here and here)
2. place in optical drive of bummed out computer, and use BIOS to instruct it to boot from disk

3. select option to 'try Ubuntu without installing it on your computer' (or whatever the exact wording is - should be the first option in most versions)

4. then wait for it to boot - the OS running from the CD can recognise your hard-drive, but won't let you automatically access it. Open a terminal and type the following (assuming you want to access drive sda1 - you may want sdb1 or something):

 sudo mkdir /media/ubuntu  
 sudo mount -t ext3 /dev/sda1 /media/ubuntu  

for ease of copying and later access, change the permissions to all directories so anyone can have read/write privileges:

 sudo su  
 chmod 777 -R /media/ubuntu  

this will take a while if you have a lot of files and directories, but when it's done you're ready to plug in a large external hard drive, and save your precious files away - exactly what I'm doing now! The next step will be to reinstall ubuntu (I'm going to go for the latest - 9.04) and spend weeks reinstalling all my software and getting it to my specification - weeks I really don't have!