Using a hash of a password as a password is great if someone is trying generic brute-forcing tactics. Where it fails (as mentioned) is against a dictionary specifically built to use the hashed values of common words etc.. If you want to use a hash as a password (which you shouldn't as you limit yourself to Hex characters), you'd be better off using the hash of a file.
For example, lets say I have a file I keep just for this purpose (keep a backup obviously), I can take a hash of this file and use it as a password without fear of someone compiling a dictionary with that in it.
Try the following using the openssl library (works on Mac OS X).
openssl md5 PATH_TO_FILE
This should return you md5 hash of the file you selected.
Additionally, md5 password hashs can be broken fairly easily. With my 2 6990s I can pull 18Ghash/s in Whitepixel (that's 18 Billion password attempts per second).