These are always more easily seen looking from the future into the past, than they are in the past looking to the future.
Few companies, with the market share RIM had, see threats as 'near term'. Fewer still have a deep appreciation for the entirety of the technology stack and the time it takes to move things. This is especially true of young executives but can happen to anyone.
RIM's technology stack matured over many years, from idea to business juggernaut. What that tells you is that moving the stack is also going to take years, so if you're experienced you start looking 5 - 6 years out not 1 - 2 years out.
When the iPhone hit, and Google was close behind, that was a huge signal. But Ballmer and Microsoft dismissing it, was a huge counter-signal. If you're an enterprise IT company, and the biggest company in Enterprise IT in the market place is dismissing the iPhone as a 'fad', you might be inclined to believe them rather than your own people who are saying "this is a threat".
At some point you get behind the power curve. In airplanes once you are behind the curve there is literally nothing you can do which will prevent you from eventually crashing. The same it true in companies. RIM apparently decided early on that Microsoft was a more credible indicator of the future than Apple/Google for their marketplace. And they have paid the ultimate price for that.