Linkage
In RSS 0.91, link was a required sub-element of item. In today's scripting news's RSS feed, link has been replaced by guid. This SourceForge feed has guids that are not links. In Archipelago's RSS feed the permaLink is encoded in the description, and the link is used for something else.
On my weblog, you can retrieve this blog entry by id, or by date. In Joe Gregorio's weblog, you can retrieve blog entries by title. Mark Pilgrim adds a date.
It is OK to have multiple "unique" ways to identify a resource. When I have enough spare moments to think through the ramifications, I'll support titles in my URLs and probably make that the preferred external view.
Now let's standardize and simplify. A well formed blog entry will have two universal resource identifiers (URI's) associated with it. One, a permaLink, will identify the preferred external "name" for this blog entry. The other, a postId, will identify the preferred internal "name".
Both can be http:// style URLs. Neither are required to be URLs at all. They can be URN's and simply identify things in a location independent manner. You can tell the difference by looking at the scheme (the portion of the URI before the ":").
I suspect that most permaLinks will be http:// style URLs.
The permaLink and the postId need not be different. In fact, they will often be the same. But let's not introduce complex precedence rules or require the recipient to guess for the sake of saving a few bytes of bandwidth in the cases when they are.
So... let's call a permaLink a permaLink and a postId a postId. Require them both to be present. And make them both URIs.