Assuming there is a distinct geek personality type, what is it? As a quick short-hand, figure it's everything pop-psych usually associates with left-brain thinking: linear, logical, analytical, and so on.
My own view is that the geekiness that characterizes IT is due less to personality type than to IT professionals being embedded in a distinct, "geek" culture.
The mathematics of personalty clashing
No matter how strongly anyone thinks that "geek" is a personality type, nobody would claim that everyone in IT is a geek, or that everyone outside IT isn't a geek. For that matter, it's doubtful anyone would consider geekiness to be an all-or-none proposition: If geekiness is a personality type, it's a continuum, with über-nerds on one end of the scale and performance artists on the other.
Let's imagine, for the sake of argument, that two out of every three IT professionals have a lot of geek factor in their personalities, while only one out of every four non-IT employees does -- quite a strong correlation between personality type and profession; the numbers aren't likely to be more extreme than this.
Given these numbers, two-thirds times three-quarters ... half ... of all interactions between IT and non-IT employees will be between IT geeks and non-IT non-geeks.
That's enough to present a challenge, but not so overwhelming as to characterize the whole business/IT relationship.
Geek culture: The greater issue
Compare this to the cultural diagnosis.
Culture has been defined by lots of different people in lots of different ways. My own consulting company borrowed its definition from a branch of anthropology called ethnoscience, which defines culture as "the learned behavior people exhibit in response to their environment."
An interesting bit about "their environment" is that in a business setting, most of the environment each employee works in is the behavior of the other employees they work with. It's a self-reinforcing loop, which means that unlike personality, which varies a lot from one employee to the next, culture is shared. Those who participate in it acculturate ... they converge in how they respond to different situations.
So while even with some fairly extreme assumptions, personality-type-driven clashes are likely to be the cause of conflict in no more than 50 percent of all IT/non-IT interactions, a clash of cultures is built into the fabric of things.
And make no mistake: IT does have a distinct culture. It has to, because in order to succeed in the work, all IT professionals have to approach their work with a certain mindset that's different from how employees who plow the fields of marketing, sales, manufacturing, human resources, and even accounting and finance think about their responsibilities.
Dealing with culture clash
As do employees who work together anywhere, those of us who work in IT develop a shared vocabulary, a set of largely unconscious shared assumptions, and shared patterns of thinking. These shared cultural traits streamline and simplify our communication. A common culture makes IT more effective.