Backing up wallet with pen and paper, destroying electronic copy, restoring from keyboard -- how?
The Satoshi bitcoin client has a dumpprivkey command available in the console, which outputs an encoded string of about 50-60 alphanumeric characters.  That should be pretty easy to write down, and it can be reimported with the importprivkey command.

But if you're really paranoid, why ever have your private key on a computer at all?  Flip a coin a bunch of times to generate some random bits, and then run the ECDSA keypair generation routine with a paper and pencil!  Hash the public key (you can use a computer for that part) to get your address, and transfer all your bitcoins to that address.  If you want to spend them someday, run ECDSA's signing algorithm by hand to make a raw transaction, key it in, and put it on the network.

Of course, stealing the piece of paper where you wrote down the key is probably a lot easier than decrypting an electronic copy.  It seems to me your trust is a little misplaced.