Tom Coates has posted a very, very good essay on permalinks and what they mean.
It may seem like a trivial piece of functionality now, but it was effectively the device that turned weblogs from an ease-of-publishing phenomenon into a conversational mess of overlapping communities. For the first time it became relatively easy to gesture directly at a highly specific post on someone else's site and talk about it. Discussion emerged. Chat emerged. And – as a result – friendships emerged or became more entrenched. The permalink was the first – and most successful – attempt to build bridges between weblogs. It existed way before Trackback and I think it's been more fundamental to our development as a culture than comments… Not only that, it added history to weblogs as well – before you'd link to a site's front page if you wanted to reference something they were talking about – that link would become worthless within days, but that didn't matter because your own content was equally disposable. The creation of the permalink built-in memory – links that worked and remained consistent over time, conversations that could be archived and retraced later. The permalink stopped all weblog conversations being like that guy in Memento…
(via Dan Gillmor)