Like most of Gladwell's ideas, it falls apart under the least examination. Consider a neurosurgeon. Perhaps after ten thousand hours of surgery she was an expert in surgery. But perhaps only a quarter of that time was spent performing neurosurgery. Does that mean she's not an expert neurosurgeon? Perhaps, then, after ten thousand hours of neurosurgery she has become an expert neurosurgeon. But maybe only 5% of that time consisted of excising glioblastomas from the anterior cingulate cortex. Does she need to spend ten thousand hours doing ACC glioblastomectomies in order to be an expert at that task?
In short, because what we normally mean by "tasks" or "fields" can be broken down into smaller tasks or fields, the statement is self-contradictory if they mean what they normally means. But if we search for a definition of "task" that makes the statement true, we can trivially find a circular one: a "field" is something that a talented person can achieve mastery of in about ten thousand hours of practice.
Once reformulated that way, it becomes clear that this is not a statement about human expertise or expert performance, but about the economic structure of current society, and in particular our degree of specialization: most people (within the professional class Gladwell finds interesting, anyway) work in jobs that require about ten thousand hours of focused practice by a talented person to reach mastery.
(I should point out that this occurred to me when I saw Jeff Fox (RIP: http://groups.google.com/group/comp.lang.forth/browse_thread...) claiming that the research Gladwell summarized meant that you needed about ten thousand hours of programming in Forth to be good at programming in Forth, so you shouldn't put too much stock in the opinions of experienced programmers who didn't have much Forth experience.)
Once stated this way, we can formulate interesting hypotheses about the cause: perhaps when a field starts to require substantially more than ten thousand hours of practice to master it, its practitioners start to specialize in a part of it, so that they can reach mastery sooner.