[jcon321]

All I know is government contracts, but I've been pretty successful at estimations. Much like the top posts in this thread, none of it is for free. Most of the time requirements come in a few sentences, maybe a paragraph, rare cases a document. There's no estimation until the requirements are flushed out in some type of use case which has some bearing on the technical constraints of the system. (The client pays for this, and provides feedback.) The next step is planning, which is where estimation comes from. (The client pays for this.) In terms of planning I do strive for maximum 16 hour chunks which helps accuracy. For estimations that I have low confidence in, I'll have conditional chunks that may be used. All of this is transparent to the client.

If the client asks for an estimation before the above steps it's known by everyone that it has no official meaning, because everyone is aware of the above process.