As Paul Graham has said many times -- ideas are worthless. Y Combinator wants to know about the capabilities of the founders. Capable people are more likely to produce capable startups. Second, I assume they want more detail on a particular idea not necessarily because your idea is OMG $uper AwesomE!!1 but in order to show that you have the capability to flesh out an idea more thoroughly. Can you think through the various aspects of your idea and their business ramifications, or are you just holding up your idea like a cardboard facade?

Well, there's all this talk about applications being judged on the quality of the cofounders and the idea not being too important. A friend of mine applied this summer and was essentially turned down because of the idea (primarily because there was competition in the area). So maybe the idea isn't that irrelevant and applying with more than one idea might not hurt.

I just put my other ideas into the box at the bottom where it asks me for "...other idea's you considered applying with..".