hello
I think Kiilo meant to compare the relative stability of the traditional instruments to the way we are still working today with electronic devices. Theremin may be still in use and have influenced many other interfaces to electronic devices but sure it is not a well established instrument.
it is not really a well established instrument, it is true. anyway, there is a list of more than 400 bands and musicians using the Theremine
http://www.thereminworld.com/bands.asp
among them, Jarre (Oxigene 10, 1997) and Spencer, as has been said, but also John Zorn and many other well known musicians.
Moog is selling one and it is MIDIfied...
http://www.moogmusic.com/cgi-local/SoftCart.100.exe/products_t.html?L+scstor...
an interesting thing about Theremine that I don't know if it has been transferred to other newer electronic devices is its highly developed technique of playing...
jmb