The revised FTC’s Guides Concerning the Use of Endorsements and Testimonials in Advertising require that blogger who receives cash or in-kind payment to review a product must disclose the material connections they share with the seller of the product or service. The disclosure must be "clear and conspicuous", no matter what form it will take. Violators may be subject to injunctions or fines. A blogger who receives a freebie without the advertiser knowing and reviews it on her site would not violate FTC guidelines. Read more details here.
I've noticed that a lot of reviewers have already been following these guidelines. Personally I think the disclosure requirement is fair and reasonable and I'm happy to comply. If you're new to blog reviews or haven't started disclosing what you receive in exchange for a review, you may want to keep these new regulations in mind when you write your next review.