Yeah, using shottkys as “clamping diodes” is a common pattern (see e.g. the BAT42s and 1N5817s mentioned earlier). They have a low forward voltage, so the input signal won’t get very far above or below the supply rails before it’s shorted away. Zeners have a somewhat different behaviour, though; they’re used backwards, and open when the reverse voltage exceeds a certain voltage.
(but I guess I just answered my own question – with shottkys, the one connected to ground will start conducting when the input voltage goes below −0.3 V or so, but with zeners the diode in the same position will start conducting when the input voltage goes above the zener voltage.)
(Note btw that the Arduino pins have internal clamping diodes, so if you can limit the current you can just rely on them. External ones are perhaps more robust, and definitely easier to replace, though 
)
EDIT: Below −0.3 V, that is – i.e. one shottky diode drop from 0 V. Similar in the other direction.