I've run across this hint a number of times now, haven't tried it yet, but I see no reason why it shouldn't work fine- glass is for sure a lot harder than steel, and the edge probably is just "frosted" to a useful "grit" size. Obviously, if you do it a LOT, eventually you'll wear the edge of the glass.

You have to wonder, though- if it works well, why does no one market a sharpener made this way? A frosted glass rod could be set in something else to give it strength, and even alone, it's probably not a whole lot more fragile/brittle than some of the ceramic rods- and bound to be a lot cheaper.