URL generated by MD5 hash of "TitleOfPost 0000-00-00 00:00"
For example:
echo -n "URL generated by MD5 hash 2010-08-08 16:26" | md5sum -
makes a 32 character hex string:
22d0036808d1cf205d75ac7d538c9e0d
Make that hex string as the title of the post. Publish the post, which will "set" the URL. Then, edit the post to put in a more useful and human-readable title. The URL will remain as the hex string.
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Eh. As I think about it, my end usage here of the cryptographic hash doesn't need to have any link with the source content. In other words, I don't really care if the 32 char string is a "true" MD5 of the post's title and date. In fact, I'd be contradicting one of my primary intentions of such a scheme in the first place: to have a unique "filename" that is independent of its content...
So scratch the first part of the previous example, about generating an MD5 from some specific source string. Instead just do something like:
openssl rand -hex 16
(or use the built-in pseudorandom special devices /dev/random, etc.)
This will randomly generate a 32 character hex string that can be used.
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Update: here's how I now do this.