http://foo.edu/bar.abc

That certainly appears to be a resource: it has a URI and when
dereferenced it returns a representation (or, more likely, a
"variant" in RFC-2616 terms).

Now I show you these URIs:

http://foo.edu/bar
http://foo.edu/bar.abc
http://foo.edu/bar.xyz

and tell that the last two URIs identify possible representations
for the 1st URI.  That means "http://foo.edu/bar.abc" is both a
resource in its own right *and* a representation for another
resource.  Effectively, its both a 1st and 2nd class citizen.  RFC
2616 doesn't directly address this apparent inconsistency of URIs
identifying both a resource and representation.  Its perfectly
obvious and reasonable from an implementation point of view, but
its really non-sensical from a modeling point of view.

The AWWW + friends introduce the concepts of "information resource"
(i.e., web things like html and pdf) and "non-information resource"
(i.e., real world things like cars and sandwiches) and 303 redirects
(which can be thought of as a generalized version of http content
negotiation as defined in RFC-2295).  IMO, this is the opposite of
the above: it makes sense from a modeling point of view but is silly
from an implementation point of view ("so the only thing this URI
will *ever* do is redirect me to this other URI?  ok...")

regards,

Michael