“C–Inspector is a web–based application that helps you to test the information architecture of your website. By analyzing both quantitative and qualitative data collected through the remote test, you can gain insight into the users’ mental models and identify possible issues with labelling or grouping.”
I’ve not tried the tool myself yet, but it looks promising. The task-based approach would appear to give rich feedback on your IA. This isn’t new, though–the guys over at Optimal Usability also just recently launched TreeJack, which I got to see at the IA Summit in Memphis. It also takes a task-based approach.