iMac G5 and iSight?

Yes, you're right that magnets can hurt hard drives.

But using the magnetic iSight mount at the point on the iMac that Apple tell you to use it will be absolutely fine.

 

You would need an insanely powerful magnet to destroy the data on a hard drive. As a matter of fact, all hard drives already have a very powerful magnet inside them which moves the head back and forth and super fast speeds. The magnet inside hard drives are so strong, once you attach it to another metal, its nearly impossible to pry off with your fingers!

 

Actually, no, you don't need a powerful magnet at all. The HD magnets are shielded from the drive platters. The actual electromagnetic force used to flip bits on the platter is very weak. However, most drives are fairly well shielded, and most people don't set magnets on top of their laptops - but even a weak magnet (or EM field, like that generated by a speaker), in the wrong place could wreak havoc on a disk drive. Go hold your drive next to a bulk degausser, which isn't very magnetic but produces a strong EM field, and I doubt it'll retain much of its data.

Edit: but the iSight mount isn't going to affect the iMac HD at all. Wrong mount site for damage risk + shielded drive = no worries.