If its a job interview, as opposed to a school test, it should be "open book".
I have never had an interview that was open book, and was told the interview was over the one time I did ask. You've worked for superior shops than I have.
if you are allowed live internet connection during the interview, search google and groups.google for all sorts of previously written code and answers.
This never occurred to me to do during initial phone interviews. (I'm too old school; if I don't know it off the top of my head, I don't know it.) I'll try to do this in the future; thanks.