Hey, this happens to me ALL THE TIME too. I usually write a few paragraphs of my potental 15-to-short-stories and then return to my "novel" :)
The paragraphs are usualy fresh with ideas, but then again, when I return to the big story, I usually forget all about the short stories...maybe I'll go where the wind takes me next time and go onto a completely different tangent..
I haven't read Heavenbound, shall check it out.
-love,
Hersa