Well, I write music in my head ALL the time. The thing is, in my head, the music never stops: See, it can be a piece of someone else's, a well known piece I'm thinking about, but about 50% of the time it's a piece of my own. When it is, I always think about some rhythms that I randomly tap out, then the melody, harmony, etc all come to me at the same time, all bunched in there. It's so natural for me, I never even thought about it being unusual. I mean, I also can get the idea anywhere, so I guess that's just me!
Heckel