If you can’t afford a pretty Moog Etherwave Plus ($519) and you still want to sound like Portishead (I know it wasn’t a real Theramin) maybe Thereminator (iTunes link) for the iPhone will do the trick for you. It’s $3 and has over 200 five star ratings.
“The theremin, originally known as the aetherphone / etherophone or termenvox / thereminvox is an early electronic musical instrument controlled without contact from the player. It is named after its Russian inventor, Professor Léon Theremin, who patented the device in 1928. The controlling section usually consists of two metal antennas which sense the position of the player’s hands and control oscillators for frequency with one hand, and amplitude (volume) with the other. The electric signals from the theremin are amplified and sent to a loudspeaker.” – Wikipedia.org
For more info: yonac.com