[jacquesm]

I'd see it as a great chance to show the interviewer in the most concrete way possible that you really know your stuff.

Far from it giving me a negative impression, it gives me a positive one, far more positive than trick interview questions or aptitude tests or other nonsense like that.

Take them up on it, and take the chance to perform a reverse evaluation on how well your interviewer knows his stuff, it's a two way street!