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.
Does any journalist or columnist at CNET have to sign non -disclosure agreements to get the scoop on products or get news. Has Tom or Molly done this at all?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic