While I agree that an internship can be a great motivator (or an early de-motivator for people that chose the wrong career), I don't think this kind of extreme behavior should be encouraged. I'm not beneath doing extreme hours for a project, a few weeks ago I didn't leave the office for almost 72 hours to finish up some polishing of a product because of a sudden client imposed deadline. I did it knowing that I would then take a few days off and that it was an extreme measure to a extremely retarded requirement. It shouldn't be the rule since it can easily lead to tired and unhappy employees, and of course disgruntled employees who think they are being undervalued because they don't stay the same time the over-worker does.
If you have a very motivated employee, by all means let him do his overtime if he really wants to, but there has to be a line where you say "stop, go home". In any case, have some good luck looking for great interns. I'm pretty sure you can do wonders for all the new kids that're just starting out.