If Psystar really wants to stick it to Apple, they can declare that the Mac OS X License Agreement does not apply to them as they do not use Mac OS X, they just preinstall it.
Then they just follow up by saying that if Apple contends that the Mac OS X License Agreement applies to the purchaser, then Apple must comply with the Mac OS X License Agreement and refund the purchaser for his/her software AND hardware ACCORDING TO THE MAC OS X LICENSE AGREEMENT.
If Apple does not comply, Apple has breached the Mac OS X License Agreement, and the Mac OS X License Agreement has zero force and effect against that particular purchaser (and he can use Mac OS X anyway he/she wants).
Before dealing with Psystar, Apple should have taken a purchaser to court in order to establish that purchaser's are breaching the Mac OS X License Agreement. Not only should they have taken a purchaser to court, they should have gone after at least one pirate and prosecute them to the fullest extent of the law.
Now Psystar can claim that not only have they done nothing wrong, but that Apple has engaged in Selective Prosecution (Apple has singled out Psystar for prosecution, but has no plans of prosecuting any actual pirates or end users).
By not addressing this issue first, Apple has made it clear that it will not prosecute customers (or software pirates) for violating the Mac OS X License Agreement, causing it to have no force and effect for customers.
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My only reaction to your defence is that it's a bit of a catch-22: Although Apple may indeed be pursuing this to cause financial harm, this defence would be extremely expensive to pursue (requiring first the full case going to court & then the counter-suit) and so... .. ..

It could be expensive, but if Psystar's legal counsel know what they are doing, it shouldn't be.
If Psystar responds with a good defense, it is business as usual until the action moves forward.
The only way Apple can harm Psystar is by applying for an injunction, and if Psystar has an excellent defense, then Apple will not be granted an injunction because they have to demonstrate they are likely to win the case and the defendant is causing them harm (they have to support their claim with some sort of evidence).
Psystar just has to create an excellent defense and discredit Apple's evidence as it is presented.
Of course their is always the possibility of their lawyers deciding to challenge the validity of Apple's EULA and then who knows what the outcome will be. The strongest challenge Psystar can make against the EULA is to contend it doesn't apply to them. Of course, if they outright challenge the EULA, then it becomes a matter of whether Apple is breaking any laws with their EULA. It is nearly impossible for Apple to prove wrongdoing by Psystar if the EULA does not apply to Psystar, and it is nearly impossible for Psystar to prove wrongdoing by Apple if they challenge the EULA. So which do you think is the better bet?
Also, there is always the possibility that Psystar's legal counsel does not know what they are doing when it comes to these issues. Remember that the copyright, trademark and licensing lawyers work for the big guys (Apple) and not the little guys (Psystar).