I think this text may be valid in relation to this issue:
"A frequently asked question about <guid>s is how do they compare to <link>s. Aren't they the same thing? Yes, in some content systems, and no in others. In some systems, <link> is a permalink to a weblog item. However, in other systems, each <item> is a synopsis of a longer article, <link> points to the article, and <guid> is the permalink to the weblog entry. In all cases, it's recommended that you provide the guid, and if possible make it a permalink. This enables aggregators to not repeat items, even if there have been editing changes."
update: this is the same text as Damien links to...