OK now we have replacements for the very ordinary HTMLArea but its not yet done snd dusted
I've installed both TinyMCE and FCKEditor in zen cart. Here's my experiences in Mac OSX Tiger:
Safari 2.0.4(419) and OmniWeb 5.5(607):
Tiny MCE toolbar appears but text loses focus as you move to apply a style, so in fact you can't really edit
FCKEditor toolbar doesn't appear at all

FireFox 1.5.0.7
TinyMCE works fine
FCKEditor works fine

Opera 9.01
TinyMCE works fine
FCKEditor works fine

So unless my experience is unique then Mac users still can't use Safari/OmniWeb with these editors.