How far beforehand would you suggest? Would walking into the interview, handing them the problem, and then telling them that you'll come back in 10/15 minutes to "start" the interview be acceptable?
Asking someone to solve a problem in the waiting rooms is considerably different then letting them solve the problem in their chosen environment. Probably not as stressful as durring an interview with people watching however.
Unless they "cheat" this should give you a good idea of what they're capable of. The cheating should also show up if they cannot explain it or why they wrote it the way they did.