I see a fair amount of debate on public fora about how and when to manage the "commitment" by the various interested parties to a system design. Somewhat oddly and even counter-intuitively, you run as much risk nailing down the balance too early as you risk by delaying until it's too late.
It helps to identify the champions and get them on board early; if you don't you risk the backlash that folks will feel slighted by being omitted from the initial design decisions. If you get everybody who's invited immediately hyped-up however, you will find that some of the champions burn out before you are ready to release them: you will need their cheerleading later for positioning and they will show up in body only, not in spirit.
A sublime key to the success of a project therefore is to involve all the interested parties early, yet manage the commitment level of user and department champions so as to be able to rely upon them when you will most require their decisionmaking abilities. You need to quietly "measure" your progress and then anticipate when you will be needing their support. If you lose a key champion early, your project may be unrecoverable.