Joan Didion writes about people who keep and carry notebooks with them wherever they go:
The impulse to write things down is a peculiarly compulsive one, inexplicable to those who do not share it, useful only accidentally, only secondarily, in the way that any compulsion tries to justify itself. I suppose that it begins or does not begin in the cradle. Although I have felt compelled to write things down since I was five years old, I doubt that my daughter ever will, for she is a singularly blessed and accepting child, delighted with life exactly as life presents itself to her, unafraid to go to sleep and unafraid to wake up. Keepers of private notebooks are a different breed altogether, lonely and resistant rearrangers of things, anxious malcontents, children afflicted apparently at birth with some presentiment of loss.
Journalists always have a notebook with them — to record impressions or jot down that perfect opening sentence for a future story. It is the first dumping ground of their ideas. It's a physical artifact of their curiosity.
It's always good to see journalistic instincts in non-journalists. Yesterday I met with a technologist who, in the first five minutes of our dinner, pulled out a notebook to make notes. It was a terrific first impression on me.
Here's my post analyzing pros and cons of taking notes in an informal lunch/dinner meeting. Here's my post on the importance of capturing your fringe-thoughts. Moleskine notebooks are in-style but I prefer spiral ringed notebooks of equivalent size because I can clip a pen to it easily.
(thanks Steve Dodson for pointing out this essay)