So should death be the fifth exception? Or should life and death be together? You know, because life and death are forever intertwined. *mysticalsparkly*
But I think they should be considered as one, because of similarity when it comes to Transfiguration.
With this particular subtopic my head is spinning. I'll just throw out some stuff I know.
Death can be animated, yes? Isn't that what Inferi are? Or they're imitating life. They're blind but they knew where Harry was and wanted to drag him with them in Voldemort's cave. Or perhaps Voldemort enchanted them and put that sole purpose in them. Otherwise, Inferi could be made to do other things. And a dead body can be transfigured in other ways, like the shell (Bathilda Bagshot) for Nagini in DH.
But I think Sarah and Carole are right. The living things one conjures might fade away, just as if you cast a spell it's effects might fade (with the exceptions of some charms and enchantments, or spells that are designed to strengthen). And the Summoning thing makes sense, as well. Perhaps it is possible to do both with magic, as I don't know how Hermione would know where exactly to get yellow canaries.
And don't time and space matter in magic? McGonagall was able to Summon a plate of sandwiches for Harry and Ron at the beginning of CoS because she knew they were nearby.
And Summoned stuff wouldn't fade, but conjured things would. Everybody be careful with this in your stories.