On http://www.netbsd.org/Ports/ the Psion is listed as a suggested port.

At http://sourceforge.net/forum/forum.php?forum_id=310796 there is a
netbsd-psionw-cvsroot.tar.bz2 archive.

Do you have any more information on how hard it would be to get NetBSD 
running on a Psion 5?

Regards,

m