But my real interest in a portable OS is for hardware like the
    latest crop of WinCE machines which don't have hard disks. They
    boot from a ROM that's probably soldered to the board. How can one
    "take over" such a machine with a custom OS? It could be
    impossible, especially if the OS in ROM turns on memory
    protection.

I think this hard-wiring is intentional. Richard Stallman had an
article about it in a CACM issue a year or so ago. The architecture of
WinCE hardware and software has been seen by several people as an
attempt by Microsoft to:

(1) Wrestle hardware control away from Intel; and

(2) Wrestly software control away from alternate OSs like NeXT, BeOS,
    Linux, Solaris on Intel, etc. that were just starting to show
    signs of life back when WinCE emerged.

But this has never stopped those amazingly creative software hackers
before, has it?

-- 
Patrick Logan                 mailto:patrickl at gemstone.com
Voice 503-533-3365            Fax   503-629-8556
Gemstone Systems, Inc         http://www.gemstone.com