Here's a cartoon from a short while back from Piled Higher and Deeper.
It used to happen to me a lot more when I had a 45 minute commute to my office. All too often I'd have what seemed to be a great insight (or come up with a great turn of a phrase) in my car, and by the time I got to my office, I'd be drawing blanks.
That's why I've started keeping a small tape recorder with me. I'm no Guy Pearce, but I realize that I've got so much stuff going on that I can't depend on my memory anymore. And you never know when a good idea will hit you.