Google's new best practices document asks bloggers who receive free products to disclose so in their review and to only use nofollowed links. Furthermore, Google wants the review to be "unique and compelling," which I suppose is just a way of saying, "Don't rewrite the product description." SEL speculates that, based on historical evidence, a group of manual actions may be waiting in the flanks for bloggers who have given out dofollow links willy-nilly and ignored disclosures.
So, if you've been sending out products and only receiving nofollow links in return...expect more of that in the future. The traffic, of course, could still be valuable. And nothing beats an authentic buzz on the Internet. But for links? Even if you get 'em, the blogger you got 'em from runs the risk of being slapped for it - which means that they probably won't be good long term investments.