I've had the "pleasure" of being in this exact situation more times than I care to count. Through the years, I've met with consulting prospect after consulting prospect and been asked to provide a price and schedule for some concept so vague that the person looking to have it built has no idea what he's even looking for.
And this situation isn't unique to software. A former business partner of mine is a graphic designer. I have spent plenty of time working in his studio and have overheard him meet with countless clients that wanted a cool new logo, letterhead, business cards, website, whatever, but they had absolutely no idea what they wanted it to look like. They never hesitated to say they didn't like what he had done for them when he was left to make a guess without any real input from them.
In an ideal world, every customer (client, boss, or other consumer of your work) would know exactly what they wanted and would be able to articulate that desire clearly. Unfortunately, I don't think that world exists and if it does, I don't know anyone that lives there.
