Regardless of who masterminded the question... I think it was a valid one. The troops need more armor for their vehicles.
As long as the facts stated in the question are true - that soldiers are scouring landfills to search for metal - then it is fair game for the media to feed it to a willing soldier.
All sorts of games are played at press conferences. Reporters pitch questions to the press secretary before presidential press conferences to try to influence who gets called upon. The fact that this administration holds so few press conferences means the media has to get their questions in any way they can. If Rumsfeld held more press conferences, I would say it was unfair for the reporter to use up the time for "soldier questions".
As to coaching - why not? Administration officials are coached on phraseology before all media events, why not coach the soldier so he can be more effective. And it did the job of focusing the administration on armor. Too bad it didn't happen beofre the election so it could be shown to the whole world how little the administration cares about the soldiers in the field.
I bet the next big issue will be D cell batteries. These are the ones they need for night vision goggles, and the soldiers are always asking for them in care packages because they can't get enough through regular supply channels.