Apple, a company that Dell never saw as a rival, was 'doomed' with its bespoke computer design according to the best and brightest at Dell.
But what really put a crimp in Dell has been their dependence on Microsoft and Intel. Being constrained in responding to changes in the world (like the rise of non-Windows OS-X) by what Microsoft did, and early constraints by Intel that prevented them from capturing some server market with AMD chips, when Dell hit tapped out their ability to wring more value out of the supply chain, they lost their growth engine. And then they looked at adjacent markets, but with a clock ticking.
Trying to go too far afield into storage, briefly pumping up the music with 'boutique' computers, then committing a cardinal sin of using their customers against themselves by creating a 'lock in' strategy with Dell custom parts (motherboards, power supplies, and chassis).
So what keeps a company, when winds shift, from being nimble enough to shift with them? I wish sometimes I could go listen to the discussion that happened during the Opteron event, the Vista event, the first signs of OSX in the enterprise, the Canonical/Linux discussion. That story probably won't come out, and its too bad, I would love to read it.