[jjluoma]

If only the task that the estimate is for would not change much after the estimate has been given.

[cratermoon]

'Walking on water and developing software from a specification are easy if both are frozen.' – Edward V. Berard

[commandlinefan]

And remember - the people asking for estimates want the estimates right now. I could give you an amazingly accurate estimate of how long a software project will take - it just might take me longer to produce the estimate than it will to produce the software.