[grobbie]

IANAL and my understanding is quite limited.

But I can direct you to Hercules which is an IBM Mainframe emulator. It is open source under an OSI approved license (not GPL but rather QPL). Their problem appears not so much that they emulate the s390 architecture (I think that the Oracle v Google Java API case set rather a conclusive precedent there, in the US at any rate, but perhaps I will be corrected), but that the license terms of IBM mostly prohibit running any of their operating systems on the emulated hardware.

Of course there are exceptions and workarounds (IBM System/360 is public domain software in the US, but MMV in other territories, and some customers have a disaster recovery contract clause that could perhaps justify their running stuff on Hercules). But my point is that this project might be a good place to start when investigating the legitimacy of commercial/proprietary emulators of others' hardware.

HTH and good luck.

http://www.hercules-390.org/hercfaq.html