Card sorting in the initial stages of the project is a noble pursuit, in my opinion, and one that is bound to help you learn more about your users, how their heads work, and the problems that they’ll have with your site. Not to mention their ideas around what your content should be, and how it should be organised and what it should be called.
An IA Validation card sort happens a little way down the track when you think you know what your sitemap is going to look like, and what things are going to be called. You probably even have some draft wireframes that you’re not ready to commit to, but that you developed as you were thinking through the conceptual model for your IA and getting into the nitty gritty of the sitemap.
Once upon a time, I used to think that a card sort at the beginning and a card sort at the end of the IA scoping process was good practice. For my mind, I think that the second user testing exercise needs to be something related to the wireframes… maybe paperbased prototypes (or maybe even interactive prototypes?!), but definitely something that puts your IA into a context… a context beyond a few titles on some cards, that is.