[SmellTheGlove]

You should let them know. Ask if it's okay, but more importantly, get them your resume and let them know what you're interviewing for so that they can speak to the relevant parts of your experience.

That said, I'm an open reference for a few folks, meaning I've told them to list me as needed and I'll pick up the phone. I know these folks pretty well, and they know me well enough to know that I don't need to prep for something like that.