http://www.flickr.com/photos/rupertslunch/564672369/in/set-72157601438527393/

These are development photos from yamaha's new gizmo. Some nice detail
on how they designed the buttons.

Cool - that's exactly what I was thinking they did - e.g. it's not 
actually a transparent button but really a solid button with an led on 
both sides to give the illusion of transparency.  At least that's what 
it looks like they did.

thanks for posting this. very insightful. very familiar....

yeah it looks quite simple to diy too, i might go this route when i 
build my next midibox.