So, I will try to sum this up.
A) As for the original question: Although there are several hints into both directions, there appears to be no evidence what so ever about whether the green indicator LED of the iSight camera is actually hardwired to the camera or not. And although, until further evidence, we have to assume that it is theoretically possible to use the iSight camera without activating the indicator light, it seems to be very hard to achieve and has possibly never been done yet (Thanks, fanxAlot for insisting on the original question with me).
B) Even if the LED was hardwired, there is no reason to feel safe from being spied on, because, as Dan pointed out, if single pictures are snapped, the light only flashes briefly, which is hard to notice.
C) If one is concerned about privacy and wants to invest time to make a Mac more safe, other issues are probably more important to look at, given that
- There is still at least one microphone in every macbook (without any usage indicator) and
- Spying on the contents of the screen and hard drive is easier to do and in many cases more revealing than the camera.
- This becomes even more relevant, now that OSX Mavericks introduces a mode where the computer is life and online even during 'sleep' time.
- Talking about agencies (domestic or foreign), with access to infrastructure providers, phones are more interesting targets anyway, since they are typically always on, always near us, officially remote controllable and harder to monitor.
Thanks for the discussion so far, and if anyone finds hard evidence to resolve the original question, please still post it here!
Cheers - Heiko