If that's the actual answer, then anything else is just obfuscating the truth.
Why should we encourage lying to try to cover for the fundamental lack of knowledge? If you don't know, trying to make up numbers never makes things better.
A surprising number of people in business think this way: not that they need to make sure of something, but just that they need someone to tell them everything will be OK, and then put the blame on if anyone is unhappy. That someone will naturally be whoever is at the bottom of the totem pole.
Managing ego and heart is harder than managing a project i think.
Gerald Weinberg
http://en.wikiquote.org/wiki/Gerald_Weinberg#The_secrets_of_...
http://www.codinghorror.com/blog/2008/01/no-matter-what-they...
Asking for efficiency and adaptability in the same program is like asking for a beautiful and modest wife. Although beauty and modesty have been known to occur in the same woman, we'll probably have to settle for one or the other. At least that's better than neither.
For example, since a time estimate can trivially be converted into probability estimate for something happening on time, all of this is relevant: http://lesswrong.com/lw/3m6/techniques_for_probability_estim....
The best technique from that set, I usually find, is betting (with actual money) on results: setting up a intra-company prediction market for when your software will ship can tell you more than you ever wanted to know about how much time it'll truly take.
There are three possible reasons for an answer of 'I don't kmow'
1. Something is unknowable 2. The person being question has insufficient expertise to discover the answer 3. The person being questioned hasn't really thought about it.
You appear to be assuming that 1 is always the reason.
What I'm saying is, that at the moment someone asks you "How long will X take to bring through the software development process", "I don't know" has to be a possible answer or your whole method is corrupt.