>line is drawn with things like online RSS readers
For starters, I'd look at the Supreme Court's opinion in Grokster. Here it is in HN-suitable pseudocode. :)
IF $distributing_product_used_by_others_to_infringe == FALSE THEN
FIND_NO_LIABILITY ( "None found" )
ELSEIF $distributing_with_object_of_promoting_infringement == TRUE THEN
FIND_LIABILITY ( "You're inducing infringement ")
ELSE
IF $actual_knowledge_of_infringements == TRUE and $failed_to_block == TRUE THEN
FIND_LIABILITY ( "You're a contributory infringer ")
ELSE
IF $capable_of_substantial_noninfringing_uses == TRUE THEN
FIND_NO_LIABILITY ( "None found" )
ELSE
FIND_LIABILITY ( "You're a contributory infringer ")
END
END
END
How this applies to online RSS readers is left as an exercise for the reader...
UPDATE: A copyright lawyer friend points out via email that I neglected to include vicarious liability. I'll leave the above pseudocode as-is and note vicarious liability here as a bugfix/todo.