very cool phenomenon. i know a synaesthete personally, although his form involves only visual perception. he percieves colors whenever he sees numbers. the associations are constant, and absolutely inseperable... he says it just "seems wrong" whenever he sees that number in a different color.
i did a bit of research on the subject for a class i took on consciousness. most cases involve vision and hearing, but there are accounts of really strange forms. a couple of examples:
taste-touch: the taste of mint evoked the sensation of touching a smooth glass pillar
sound-form: certain words are associated with spatial forms, which the subject tried to reproduce by contorting his body.
