It really depends on what you need to sense - and how accurately.
A Sharp IR etc may detect a change in reflectance.
I guess you are against my 'home made' solution - and that's fine - but take a leaf from the 'blind' community (of which I have no direct knowledge and so I apologise to them in advance for any over simplification) - but they tend to have a 'white stick' feeler - and not a fancy electronic thing. Their stick is 'binary' - ie 'ok' or 'problem'. And surely an analogue signal, from a Sharp say, would need to be the same - ie 'Im only 40% falling off the table' doesn't mean much.
Of course your main objection may be having a mechanical 'feeler' device mounted on the bot. In which case a photo sensitive device could be a cure - so long as you can live with wrong readings eg where dark = void