Uniqueness
It seems that both Blogdex and KeepMedia seem to think that they each have exclusive rights to the domain of small positive integers. I wonder what part of Globally Unique Identifier people have difficulty understanding? Seems clear enough to me.
What happens when guids collide? Well, at least one popular aggregator will decide that you have already seen the post.
Because of this, the feedvalidator will now flag both of these feeds as having guids which are NotSufficientlyUnique. Thanks to Joseph Walton, such messages will be clearly marked as warnings and not cause the feed to be flagged as invalid.
While neither RSS 1.0 nor Atom can absolutely prevent collisions, both rdf:about and atom:id are defined as URIs. This means that such identifiers (by virtue of not containing a colon) will be evaluated as relative to the source page. Which means that they only need to be locally unique.
Furthermore, both rdf:about and atom:id are defined as being required. Which is a good thing.