It’s particularly worth reading for its great explanations of content negotiation and of the difference between information resources and non-information resources.
(Sorry for that bit of web architect jargon, but look at the presentation and all will become clear).
Ric mentioned this issue in his What is Linked Data? post the other day, noting that we have one URI for the City of Manchester local authority itself, and another one for a web page about it.
Their key point is:
“We want to be able to make different claims about the thing and the document about the thing”.
Yves and Michael explain very clearly why this is important, with lots of good examples. But enough from me – just go and read it.