Thanks for the advice! So I can get my head round this:
pwillard:
... the forward biased zener will add a .7v drop to forward voltage... in essence leaving you to shunt excess voltage that must first reach a potential of 7.5V before shunting occurs... which already exceeds maximum pin parameters.
Understood - seems to be down to my clumsy selection of 6.8v zeners. But if I used 4.7v zeners instead, the voltage would only hit 5.4v before shunting.
Using Schottkys in the configuration you suggested would protect against voltages over 5.33v (assuming BAT-43s with a .33v drop), which is only half a volt-ish more protection than the zeners but with double the component count - is this enough of a reason to use your 8-diode method over my 4 zeners? Is there another reason not to use zeners?
dc42:
Have you measured the resistance of a stepper winding? If it's less than 80 ohms then you really should use a driver.
Resistance of the windings are around 270 ohms if I remember right - oh, which would mean around 18mA draw per IO pin.
MarkT:
Ooh, can we see picture or link to these tiny steppers?
Two types I'm trying to drive with minimal extra circuitry. The first one (not out of a camcorder, d'oh - got my steppers confused - it's from a cheapo Chinese electronic rev counter dial) :
All it has to move is a tiny plastic needle indicator. Inside, it's more like a watch mechanism than a normal stepper:
Two coils, some metal plates to carry the magnetic field (I really don't know what I'm talking about but it all sounds plausible), and the tiny little black cog in the centre is magnetic.
It was a bugger to get back together the first time 
It works really well, though; it's surprisingly strong and with the help of the Arduino AccelStepper library it's fast, accurate and repeatable too.
If I can drive them without killing the ucontroller, these are the camcorder lens motors I want to drive. They're like miniature floppy drive head motors. The photo makes them look way bigger than they are in real life. Dinky, huh
Atmega168 TQFP on the left, 0805 resistor below. (Old obsolete key, feel free to make copies - they won't let you into my secret workshop 