I stumbled across the Einstein Newton emulator the other day for Mac OS X or Linux PDAs. You need to run some software on your Newton to extract the ROMS before using it. Here it is running on a Nokia 770, a Linux PDA (Sharp Zaurus?) and something else, I’m not sure what.
There’s an interesting comparison of the Nokia 800 user interface and the Newton’s here. Personally I think the Newton was well ahead of its time, it still has a better user interface than any Palm based machine, great handwriting recognition, and better integration than Apple’s currently got in all its personal contact apps put together (AddressBook, Mail, iCal), and indeed, across its entire operating system. It will be interesting to see how the iPhone compares, but 10 years on the NewtOS still provides better features than the competition.