Yes, CNET product reviewers sometimes sign NDAs to get info or review units early. Molly wrote a great column about this awhile back and stated why she thinks NDAs shouldn't be signed.

http://www.cnet.com/4520-6033_1-6214591-1.html?tag=txt

I have only ever signed one NDA and it wasn't while I worked at CNET. It was related to a Web service provider we were contracting with at TechTV.com, not a product we were reviewing.