forgive what is possibly a very ignorant question, but are there security concerns here? I understand that the key space is immensely huge and that for any file over 4MB in size it would be virtually impossible to guess, but what is to stop someone from just trying hashes for fun to see if they get interesting files?

Like I said for file over 4MB it seems fine, guessing sequential hashes would be all but impossible. I assume the realistic solution is just to encrypt my files (preferably in a truecrypt volume over 4MB in size) if I'm truly concerned.

On a side note, it would be interesting to see if this could be modified to tell me how unique my overall file set is.