I thought of a simple way to implement the timestamp concept I mentioned above. Run sha1sum on the file you want to timestamp. Convert the result to a Bitcoin address, such as via http://blockexplorer.com/q/hashtoaddress . Then send a small payment to that address.
The money will be lost forever, as there is no way to spend it further, but the timestamp Bitcoin address will remain in the block chain as a record of the file’s existence.
I understand that this is arguably not a good use of the Bitcoin distributed database, but nothing stops people from doing this so we should be aware that it may be done.