As a mercenary engineer, I've had to answer the estimation question many times. Whenever I've given estimates, it has only ever shot me in the foot. Spolsky might have figured it out, but I haven't. So I try to avoid giving estimates as far as possible and instead focus on demonstrating velocity of a working system they can choose to stop funding at any time.
Some clients, especially from the construction industry, expect me to give them GANTT-chart style estimates down to the hour. Here is how I convey the software estimation problem to them:
If I write the same code twice, I done fucked up. So any code I write, is new code. If I can reuse code, I will, but that's not the code you're asking about. In your industry, you have built the same house many times before with the same team, on the same foundation. But you are employing me to design, architect and construct a new house that has never been built before. If it existed already, you would just buy it off the shelf instead of employing me.
So think of me as more of an architect than a labourer, and together with you as the client, we are embarking on a design that you're not quite sure you want yet, that won't collapse on a potentially unstable foundation.