Your way of interview is of course a valid one. Instead, I've seen too many companies hire poeple just by blah, blah and end up with a guy can just blah, blah.

However, it's based on a major assumption, that is: you just want to hire a normal person who can do coding. Because the job you give to the interviewer is too simple. Of course, you don't want to hire a person better than you.

There is a thread in www.theserverside.com on "how to interview a programmer?", but that's for hiring a good person, not suitable for you.