Firstly, I agree with Phil, Branson's cited quote dealt with personal opportunities, not projects. So to a major extent, the quote is out of context as far as project management goes. But, given the typical customer response, they prefer to hear "yes" to requests. My mentor said it like this, "Learn to get smart quick if you don't know anything about it." With 40+ years as project manager/director/implementer I usually will say "Yes, it can be done, but given the risks and unknowns the timeline will be between n to x days (or weeks, or months)." Often meeting a deadline in a project plan depends very much on the quality of the team in conjunction with the project manager's effort estimating ability. In other words, I nearly always commit, but my time frame can become very fuzzy depending on the request.