Here’s a great quote from the interview at Boxes and Arrows: Talking with Jesse James Garrett

People get hung up on specific techniques too easily. Look past the technique to see the problem the technique is intended to address. If “doing IA”means diagrams, nav specs, wireframes, then no, you don’t always have to do IA. But if “doing IA” means thinking about the structure of your site, then absolutely, you need to do IA every time.