Why is the curse of original sin not specific to humans?

That's a loaded question... it is basically "the problem of evil"... let me give you my understanding:

The story of Adam and Eve is meant to be an illustration of a mystery we cannot comprehend - so the description that "due to their (our) sin, the world was created this way", is an approximation.

From theological (not philosophical) perspective - The world is a GIFT to man. How can such a terrible world be a gift? We trust and are told that it is.

Consider this:

"[3:22] Then the LORD God said, "See, the man has become like one of us, knowing good and evil; and now, he might reach out his hand and take also from the tree of life, and eat, and live forever" [3:23] therefore the LORD God sent him forth from the garden of Eden, to till the ground from which he was taken.

Sounds pretty cruel does it not? :)

God casts us down so that we cannot be "like-Him", presumably "eternally happy" and live forever?

So either God is bad, or, if not, then this was a good thing for us. How could we understand this?

One possibility: We were given THIS world as a gift because only this world can help us reach salvation; unlike satan who lives forever with full knowledge - ie, does not grow, does not learn, cannot be saved.

We do learn / grow; this world is exactly the world needed to direct our learning to salvation. Perfect world for this goal. In other words, if we did have full knowledge, we would ask for this world, exactly as is.

So, "Is this fair and just?"

It is MORE then just fair and just, it is an unbelievable gift. It is beyond justice, it is mercy.

Not a very "satisfying" answer I am sure, nor a philosophically defensible one, but I think consistent with Catholic theology.

So, if we agree that the world is exactly as it is needed for our salvation, and a gift, how can we justify animal pain, for example.

I have never seen a truly satisfying answer to this. Most explanations go something like this:

We do know, that the world was created for us. Everything in it, is meant for us. The suffering of the world, is needed for our salvation. So in a terribly politically incorrect way, the world without man, is meaningless; so the suffering of the world, is inconsequential without man(1).

The entire universe was created just to teach man a lesson, so to speak...everything in it, is for this purpose. That is a mystery, it cannot be explained logically or philosophically. All we got, is the above logical explanation that falls short of total explanation, and we are left with "trust".

I have also seen a number of theologians try to explain it, by belittling animal pain (and there is some truth to this, see below (2)) but it is not a completely satisfying explanation and it is usually ended with "we must trust its good and just". See http://www.youtube.com/watch?v=UU7sqi8iBBI for John Hare's view on this.

Is it therefore illogical, or contradictory as atheists often claim?

No. contradiction would require full knowledge and we recognize that we do not have it.

I highly recommend JPII's look at this question in http://www.amazon.ca/Man-Woman-Created-Them-ebook/dp/B005F1Q7EW/ref=sr_1_1?ie=UTF8&qid=1370717893&sr=8-1&keywords=theology+of+the+body

There are also lots of good (long) videos on Theology of the Body, for example, I recommend Fr. Thomas Loya, http://www.youtube.com/watch?v=NkYXLV_8aJ4&list=WLF4AEF5D2356584FB

JPII's focus on the body and what is means to theology, is exactly the answer to your question.

(1)Note that the atheists claim the EXACT opposite - that the MAN is inconsequential to the world. That the world exists for its own sake, that we are just an accidental development. That the world was first, man a second thought at best. It is this flip that makes it impossible to reconcile the world with theology. It is a trick, designed to give us false information, so that our rational side, that is designed to recognize God, is confused.

(2)consider, what is true suffering?

Being in pain now? Or being aware of the long duration, or unending of the state of pain?

We know fairly well, even scientifically, that immediate pain, is bearable if we know it will end (consider laboring woman who is told its almost over, or root-canal work when the dentist tells you 2 more minutes and it's done).

Since animals and plants do not have such cognition, it is very hard to determine how much they are really suffering. I am not condoning animal cruelty! but its easy to anthropomorphize animal suffering.

Also consider the importance of "remembering", in a self reflective way. As anyone who had a little-too-much-to-drink can tell you, hearing that you had fun, or were in pain last night, is really meaningless, experientially. It is also hard to determine what and how the animals remember.