> Easiest way is simply XOR the database auto-increment ID with a fixed number, then truncate to (AND to) however many bits seem reasonable for your site.

XOR has a trivial known-plaintext attack, and you don't even need to determine the entire key to get something useful. If you can create a few posts in order and obtain a series of sequential items, you can obtain the low-order bits of the number XORed in, at which point you can easily browse around for adjacent items from any post.