To the list of platforms supporting Unicode, you may add the AlphaTk text
editor (http://www.santafe.edu/~vince/alphatk/about.html). As it is written
completely in Tcl/Tk it is available on essentially any platform to which
(a sufficiently new version of) those scripting languages have been ported
(this includes UNIX and Windoze), although practical use of Unicode is
probably further conditioned by that the platform GUI can handle it. I
might add that Alpha(Tk) has a very well-developed TeX/LaTeX mode.