Hi Frank,
No theremin is "perfectly linear". As with fingerboard string instruments, the distance of a semitone in the higher octaves is shorter than the distance of a semitone in the lower octaves.
Clara Rockmore used to say that modern thereminists were too "greedy" because they often wanted a six or seven octave range on their instruments. This made precision playing extremely difficult because the distance between notes in an 18 or 20 inch playing arc was too small to control accurately.
I am not sure about the new generation of Moog Etherwave theremins, but the older models (from the mid 90's) had a linearity that was extremely "pinched" the closer you got to the pitch antenna. Playing anywhere near C6 (that's "high C", two octaves above Middle C on a keyboard) was practically impossible because the space between semitones was only about a quarter of an inch. It is my understanding that the later model Etherwaves are much improved.
Bob Moog solved the problem of linearity vs the demand for more range by adding a range selection knob (low, middle and high) to the Etherwave Pro. Each of the three ranges is one octave up or down from the other. If you are a reasonably skilled player (and if you use a pitch preview) you can actually switch ranges in the middle of a piece provided there is a rest of a bar or so that will permit you to do it. This would be like a violinist being able to switch to the cello in the middle of a concerto!
A few years ago I physically measured the distances between all the notes within the playing arcs of the RCA, Ethervox, Etherwave (old model) and Etherwave Pro theremins and illustrated them on a comparison chart. I did this by stretching a piece of fine white thread from the rods of each of the instruments, attaching it to a wall several feet away, and then carefully marking the positions of the tones on the thread with a magic marker. I used an 18 inch playing arc as the standard. You can see the chart at the following URL on my website:
linearity chart (http://www.peterpringle.com/linearity2.jpeg)
You can readily see from the chart that theremin linearity and configuration vary greatly between instruments. "Configuration" refers to the number and placement of the notes within a theremin's playing arc. "Linearity" refers to the distances between those notes.