Dead Pixels

Is it a big problem with iMacs?

It bothers me a lot because of the "the screen in the computer" thing. How long do go before they usually start to die and what can you do about it?

No more a problem than it is with any other LCD screen.
It is simply the nature of the technology, sometimes you get a dead pixel here and there. It makes no difference who makes the LCD, it can just happen.