Ross, I do not dispute your arguments : I was answering Keith's question in an honest way. I (personally) do not think of a space in TeX output as a character at all, because I am steeped in TeX philosophy; but I am quite willing to accept that /if/ the objective is not to produce output for the sake of output, but output for subsequent processing as input by another program, then there /may/ be an argument for outputting a space as a variable-width glyph.
However, I do think that what appears in the output stream is a secondary consideration; far more important (IMHO) is how we represent that space /within XeTeX/. There is, I am sure, not a suggestion on the table that we start to treat a conventional space in XeTeX other than as TeX has traditionally treated it, and therefore the real question is (to my mind), "do we adopt an extension of this traditional TeX treatment for non-breaking space, thin-space, and any of the other not-quite-standard spaces that Unicode encompasses, or do we look for an alternative model which /might/ be glyph- or character-based ?". ** Phil. -------------------------------------------------- Subscriptions, Archive, and List information, etc.: http://tug.org/mailman/listinfo/xetex