Some fantastic reality imparted by 37signals.com:

So what should you do in place of a spec? Go with a briefer alternative that moves you toward something real. Write a one page story about what the app needs to do. Use plain language and make it quick. If it takes more than a page to explain it, then it’s too complex. This process shouldn’t take more than one day.

Getting Real: There’s Nothing Functional about a Functional Spec (by 37signals).