“Permalink changes”? Yes, permalinks are not as permanent as you might think. Here’s an example that happened to me. My permalink URLs were automatically generated from the title of my entry, but then I updated an entry and changed the title. Guess what, the “permanent” link just changed! If you’re clever, you can use an HTTP redirect to redirect visitors from the old permalink to the new one (and I did).
Mark is more clever than I am; I couldn’t work a good redirect solution when I wanted to change URL’s before.
Suggestion for WordPress Functionality
WP‘s Atom ID-like behavior in using the post slug to generate the permalink is a great behavior, but upon thinking further, I’d suggest locking the Post Slug field in /wp-admin/edit.php when the post’s value is set to “publish”, because a published permalink shouldn’t ever change.
[N.B.: The more I read about Atom, the more I like Atom, but I also agree with Sam Ruby that every syndication format out there has its place. The ephemeral nature of the feeds makes them unlike hard products, where convergence on one format ends up creating a de facto standard—VHS v. Beta, anyone? I like Atom, and I’ll continue to serve it, but it’s not as if I’ll stop serving RSS 0.92, RSS 1.0, or RSS 2.0 in some fit of ideological ferver.]