Value investors all saw the opportunity in AAPL, but it's nice to have a media celebrity investor like Icahn act as the catalyst to reach fair value quicker.
“Diversification for its own sake is not sensible. This is the index fund mentality: if you can't beat the market, be the market. Advocates of extreme diversification — which I think of as overdiversification — live in fear of company - specific risks; their view is that if no single position is large, losses from unanticipated events cannot be great. My view is that an investor is better off knowing a lot about a few investments than knowing only a little about each of a great many holdings. One's very best ideas are likely to generate higher returns for a given level of risk than one's hundredth or thousandth best idea.”
If you have a family and stuff to look out for, you should definitely diversify your portfolio. It can't possibly be worth the risk!
But if you're young and have no family... best of luck, man. But I am an Apple doubter and would invest like that no matter what company you're talking about.