I too think the interesting question is when is piracy okay - and for a variety of reasons, I think the answer is never.
We have a gut feeling that when a piece of software is abandoned - not available for legal buying anywhere, it's not harming anyone to download it and play it for free. After all, no harm, no foul, right?
However, a quick analogy shows how wrong this is. If a book is released in 1990, and has a very limited first run, it may very well be hard to find a legal copy in 2010. However, I don't think anyone would think that it's then okay to go out and download an illegal version of the text available online. Why the difference? I think there are two differences, which come from the relative infancy of the video game industry.
First, there's not a very well developed second-hand market for video games. Gamestop is it - and if you want anything older than last generation(and even then, the selection is terrible), you're pretty much out of luck. This, combined with the deterioration of the stuff video games come on(old floppies, intellevision carts, whatever), makes the second hand market function very ineffeciently. With a book, we believe it should be available to buy even limited, rare editions, just for the right price. With video games, this hasn't necessarily been true - but then again, with the rise of Ebay, Amazon, etc., it's more and more becoming the case. So this reason is disappearing.
The second reason, I think, is that with the rapid advances in video game technology, there's been a general feeling that those old games will never make any money for the makers anyway. Who's going to buy a new version of Secret Weapons of the Luftwaffe, anyway? Whereas with a book, as long as the book is under copyright, there will always be a market for second editions, etc. However, this basic assumption is also proving more unsound as time goes on. Retro compilations, Virtual Console services - I mean, you can get a copy of Burger Time that will play on the Wii! So these old copyrights, these old intellectual properties, are proving to be not the worthless husks video game enthusiasts once thought.
Look at an old game like Prince of Persia. Here was a quintessential dead franchise - untouched for how long. And now, several modern games and a movie later, it's proven to be very valuable intellectual property. As people more and more recognize the worth of older copyrights, I think that the feeling it's okay to download a game just because it's old will lessen to a degree. Of course, since piracy is rampant and wide-spread on current generation games which are easily available to purchase, it's obviously foolish to think that it will ever stop. I just don't think it will be as justifiable.
I know this verges on off-topic, but it's a thought I've had on my mind for a while, and this seemed as good a place as any to put it.