This is not true. The software Prey (which is designed to spy on people who have computers that have been stolen) takes covert photos and sends them without activating the light.
I just installed Prey, marked my MacBook Pro as missing, waited, and the light definitely did come on - albeit for around two seconds - while taking the webcam picture.
I did this test as well when I first heard about Prey. The first couple times when I knew the picture was going to be taken I did not see the light.
The reason is because we can only focus on one thing at a time. While waiting for the picture I was casually browsing the net -- probably reading HN -- and only periodically looking up to watch the light. I assumed a light source would catch my peripheral vision. It had not. For the third test I stared at the camera with an unbroken focus. Sure enough, the light turned on as expected.
Net: I could not see the light unless /I was looking at the camera/. It's a side effect of how the mind works and processes information. It's also important to keep this fact in mind when building interfaces: don't alert users in places they aren't looking. They won't see it no matter how red or many sparkles you add. :)
There is no way in software to disable that LED.