You will need to determine whether your network is secure, but *nobody is "hacking" your iSight*.
Check for forgotten items someone who has physical access to your Mac may have added:
• camera-capable
System Preferences.... > Accounts > Login Items,
• camera-capable widgets, and
• camera-capable applications open in the Dock.
If you don't have any of the above and have not installed, activated, and authorized
Apple Remote Desktop or some other software specifically designed to allow remote operation of your Mac, you can be confident that your webcam is not being used without your permission.
If you still see the green light occasionally, consider the reset suggested in
http://support.apple.com/kb/HT2090
You might also want to test another user account to see if the unexplained green light is user account specific.
EZ Jim
Mac Pro Quad Core (Early 2009) 2.93Ghz w/Mac OS X (10.6.4) MacBook Pro (13 inch, Mid 2009) 2.26GHz (10.6.4)
LED Cinema Display G4 PowerBook 1.67GHz (10.4.11) iBookSE 366MHz (10.3.9) External iSight