What is Switching Threshold?
In this mode of operation, it's best to think of an inverter as being a very high gain inverting amplifier.  A slight movement around the Vm point will be amplified on the output.  Indeed that is one application of inverters as analog amplifiers around a limited input range.

Whether or not it get's inverted is determined by whether or not teh follow on logic reads the input as having met their threshold.

So it's inaccurate to say it's indeterminate, perhaps transitioning is the better way of looking at it.

Here is a link to an old fairchild datasheet (warning *.pdf) that shows the curve.