AmigaGuy (and others),
All though the thread, if I am skimming with any comprehension, there seem to be lots of users confused by the difference between actually using source code, and simply supplying compatible APIs by clean-room design and developed with available documentation (of which there is plenty, as Downix points out).
Anyway, there is a huge difference legally, a huge difference in the delivered code base (and in some cases, even the OS architecture could be significantly different). And as some have pointed out, there are legal precedents.
As for whom would need to prove what in such a case, the burden of proof would fall on Amiga Inc to show in court after filing suit that IP (source code) had been infringed/used; compatible APIs are no sin or crime here. Well, Amiga blustered months back and even publicly issued a deadline (for amnesty?) but I see absolutely no signs that they followed through on anything like this.
To address another (rhetorical?) query here: the reason Genesi has pressed suit is unrelated entirely, and evidently because it seemed to be the only way to get a response after contractual agreements failed to be honored. Notice I said "evidently": it is for the court to decide. And it isn't cheap to do this for Genesi, but when parties agree to sign a contract and later one does not honor it, someone often feels it needs to be resolved.
I think it's the same as it would be for any of us, had we contracted for services or products and failed to recieve them, or even have queries about fulfilling those obligations acknowledged. I'm not sure how deep it goes and wouldn't gossip about it if I did, but that's the picture to the best of my knowledge.