Some quotes from a thread on 37s:

“How does a project get to be a year behind schedule? One day at a time.” -Fred Brooks, software engineer and computer scientist
“We release things when they are ready to be released, not based a we-can-predict-the-future schedule.
Priorities shift, products change, new ideas bubble up, we discover new techniques and concepts, mistakes are made, external circumstances reveal themselves.
All those things make schedules a waste of time. They don’t account for surprises, new opportunities, gut feel, and human error. Schedules are too theoretical for our tastes.
The only time we start thinking about dates are when we’re really close to release. Then we can say “let’s try to get this out next Monday” or “Let’s do what we can over the next couple week and then go live with it.” Our schedules are relative.