Unfortunately, beyond the similarity of the ARM processor, which despite being descendent from the one used in the MessagePads is probably a slightly different instruction set, the MessagePads and eMates also used a significant amount of custom chips as well. The problem is that the whole architecture is different, so you wouldn't be able to run the NewtonOS directly in modern hardware.

That said, Einstein emulates all that custom hardware, including MMU (memory management unit), allowing it to run under other operating systems. I don't think it has been done yet--and I could be wrong about that--but I know one plan was to support passing ARM instructions through to the processor (I believe some of the funky MMU work had to be handled first) so as to reduce some layers of emulation to make it run faster. The Einstein developers are working in removing other layers, incl. much of the ROM, so it's getting better and better.

Ultimately, Einstein really is required to run NewtonOS on other platforms, regardless of OS. I'd love to see someone develop a minimal Linux/BSD based OS install with just enough to run Einstein on it, further reducing the overhead for running NewtonOS and making it feel more like a native installation.