It's called an Ondes Martenot. It's an early electronic instrument from the 20's
>> ^gnargnar:

what the hell is greenwood playing? is he giving it vibrato via finger wobbling, or is it just a bad habit like i have?

He's plaing an Ondes Martenot. He plays one in songs such as How to Disappear Completely and a lot of other post OK Computer songs.

In reply to this comment by gnargnar:
what the hell is greenwood playing? is he giving it vibrato via finger wobbling, or is it just a bad habit like i have?

Star Trek was the first time most of us heard a theremin -- though we didn't know it!

[edit] Turns out I was wrong -- it was an Ondes Martenot the first season, and a soprano named Loulie Jean Norman in the 2nd & 3rd seasons.

[re-edit] Hmm, the Martenot thing may only be a rumor.... very mysterious!