Rob, the 'problem' using a standard opto is that they only barely switch low enough to toggle the switch line below half way so that the micro can 'see' that the switch has gone low.

To clarify:

Normal rail voltage is 5V
The micro 'sees' a low when the voltage falls below about 2.2V - anything above this is seen as a high.
Standard optos will switch down to (anywhere between) about 1.8~2.6V
If you are lucky (as you are in this case) you have a part that is switching below 2.2V and so it is working

This all depends of the batch and down to the individual part. Chances are if you tried another 10 parts from Jaycar, from the same batch - some might work, some might not???

In the long run you could never be certain that it will keep working as it is right on the edge....

I hope that makes sense???