These are the ones I use. These are for when one of the 807 people following you on Twitter ask to have coffee to tell you about their new application which is like FaceBook crossed with Chumby but better...
You get the question, and what I'm looking for thrown in for free.
1. What is it? Simple
2. Who is it for? Targeted, identifiable
3. Why do they need it? Pain, valuable
4. What is the market like? Validation, gaps.
5. Why will this win? Unique, defensible.
6. How does it make money? Clear, real
7. How big could it be? Huge, high margin
8. Who is the team? Committed, done it before.
Now you have all you need to snap some good questions back and keep writing away on that napkin.
P.S. No matter even if the plan is the suckiest stupidest idea since pets.com always always encourage, support and tell them to go for it if their heart believes in it. Much better a passionate failure than a life of quiet desperation and regret.