I was reading the January 2010 issue of Poets and Writers today and came across an excerpt from the book Is Life Like This?: A Guide to Writing Your First Novel in Six Months by John Dufresne.
Dufresne really hit the nail on the head with this paragraph on where ideas for novels come from:
"Where do you begin writing a novel? At the desk of course. And how do you begin? The idea for a novel might come from the events of your life. Examine your past. Write about what hurts, what broke your heart. Write about what you don't understand. Write about what you can't forget. Write about your regrets and your outrage. A novel might come from the events of someone else's life. A novel might be inspired by what you've read, in fiction or in a newspaper. It might begin as an anecdote overheard or a snatch of dialogue from the folks at the next table in the restaurant. Novelists are inspired by whatever attracts their attention, by what pops up in their notebooks. A novel might begin with a strong emotion, a character or a situation, a place, an overheard line, a provocative image. In short, a novel can begin anywhere, with anything, so long as it fascinates you, worries you, or makes you wonder."