[Images associated with a research experiment involving synaesthetes. On left, the the image of a number matrix as the non-synaesthete sees it; on right, a color-coded representation of how a synaesthete might see it. Ramachandran & Hubbard, "Window" (full citation below). FYI, I immediately see the triangle of 2's when I glance at the image on the left - except for me the colors are reversed: 5 is red, 2 is green.]
What I find so interesting about the discourse of Ramachnadran and others is this insistence that synaesthesia is not "just a metaphor." We all use phrases like "loud shirt," "bitter cold," or "sharp cheese" and automatically understand such phrases are instances of cross-modal metaphor, a mere trick of language.  That is, we all know a loud shirt is not actually making any sound, for instance, and we are metaphorically employing one sense (sound) in order to discuss another (sight). But what if one's sense experience is effectively already cross-modal? What happens to metaphor then? Ramachandran & Hubbard have their speculations: if "[s]ynaesthesia is a concrete sensory phenomenon [with] neural basis" (R&H, "Window" 4), then the (possibly) "more cross-wired" brains of synaesthetes might suggest "the neural basis of metaphor" itself (R&H, "Window" 28). Metaphor, R&H might suggest, is at its core a type of cross-modal thinking; one must always think in one modality while activating another.
[St. Cecilia refuses to kneel before a statue of Jupiter. Domenichino, "St. Cecilia before the Judge." Fresco, San Luigi dei Francesi (1612-1615).]
 Sajiv, Noam & Ward, Jamie (2006), "Crossmodal interactions: lessons from synaesthesia." Progress in Brain Research 55: pp. 259-271 [PDF].
 Cytowic, Richard E. The Man Who Tasted Shapes (Cambridge: MIT Press, 2003). See also Cytowic, Richard E. Synaesthesia: A Union of the Senses. 2nd Edn (Cambridge MIT Press, 2002).
 Ramachandran, V.S. & Hubbard, E.M (2001), "Synaesthesia: A window into perception, thought and language." Journal of Consciousness Studies 8, 12: pp. 3-34 [PDF];Ramachandran, V.S. & Hubbard, E.M (2001), "Psychophysical investigations into the neural basis of synaesthesia." Proceedings of the Royal Society 268, pp. 979-983 [PDF].
 Some of these same cross-modal metaphors are listed as examples by Ramachandran, V.S. & Hubbard, E.M. (2003), "The Phenomenology of Synaesthesia." Journal of Consciousness Studies 10, 8: pp. 49-57, at p. 52 [PDF].
* P.S. Just noticed an error above: it's "grapheme-color" synaesthesia, not "color-grapheme" synaesthesia. I've kept the strikethrough here so you can see the error -- my transposition of words (a typing equivalent of a Freudian slip?) suggests how intertwined my experience of letters and colors really is.