The mantra of User Centered Design is "involve users throughout." To a manager concerned about time, budget, and the project plan, this can seem like a logistical nightmare. For internal application projects this can mean business analysts and interface designers are directed to get requirements by proxy from end user's managers. Too often, this approach leads to analysis and design failures not caught until user acceptance testing at the end of the release cycle.
It is paramount to leverage the "what's in it for me" factor to engage end user's at the beginning of the process. When end users feel ignored during the requirements phase they approach UAT as their only opportunity to voice displeasure with the development process instead of focusing on constructive feedback geared at improving the application they will have to use on a daily basis.
It is a good idea to balance stakeholder requirements with top down interviews, starting with the most senior business sponsor of the development effort. If a core team approach is going to be utilized to drive analysis, design, and feedback, cycle in the different levels of business stakeholders on group at a time. Unlike a JAD where all stakeholders openly discuss their varying needs, a core team should focus on one needs set at a time. I have found when end users and their bosses are in the room together there can be too much emphasis on maintaining a "happy family" atmosphere and honest feedback is pushed aside.
There definately are business realities that make extensive end user involvement logistically or economically unrealistic. For product development efforts, time to market pressures or investor expectations time box the entire development effort. In these cases, I reccomend defining Personas and User Scenarios in the context of "Release X Assumptions" such as, "our main customer is the typically family: (1/2 parents 2 kids)", "our secondry customers are single professionals/ roomates." The focus in this approach is to initially narrow the development effort and react quickly once user feedback starts coming in.
The bottom line is this: User Centered Design needs to fit within the corporate culture. Some cultures emphasize the effort up front while other prefer to react quickly with many flavors in between.
